Add iOS toolchains, platforms, and constraint_values

[jayconrod]

Refactored the description of target platforms. The source of truth is
now the PLATFORMS table go/private/platforms.bzl. Declarations for
config_settings in //go/platform; constraint_values and aliases in
//go/toolchain; and toolchains in @go_sdk are generated from this
table.

In addition to the normal GOOS_GOARCH list, there are some additional
entries for iOS. iOS platforms still have GOOS=darwin, but they are
compatible with a different constraint_value
(@bazel_tools//platforms:ios instead of @bazel_tools//platforms:osx).

Fixes #2079

[jayconrod]

@steeve Please take a look. This should make it possible to bazel build --platforms=@io_bazel_rules_go//go/toolchain:ios_arm.

This is incomplete. I don't have a solution for select yet. Although it seems to compile correctly, it won't link (but my toolchain is probably just messed up).

[jayconrod]

I added a commit that should fix @io_bazel_rules_go//go/platform:darwin. It should be true if target platform is any Go darwin or ios platform or if the target platform is the default (not cross-compiling) and the host OS is macOS.

[steeve]

thank you!
@jayconrod it seems I can't use the @io_bazel_rules_go//go/toolchain:ios constraint with your patch. I suspect only the os_arch combination exist ?
The reason is we have our own platform that used to apply rules_go constraints.

[steeve]

I tried constraining on @bazel_tools//platforms:ios, and I'm happy to report that it works!

[steeve]

Now I'm having issues on Android:
With android_arm64:

ERROR: While resolving toolchains for target //XXXX: no matching toolchains found for types @bazel_tools//tools/cpp:toolchain_type
ERROR: Analysis of target '//XXXX' failed; build aborted: no matching toolchains found for types @bazel_tools//tools/cpp:toolchain_type

I suspect this is a problem in the NDK rules of Bazel.

And on android_arm:

ERROR: /private/var/tmp/_bazel_steeve/309ac22c92b055a3e3f78825fe63406b/external/org_golang_google_grpc/internal/syscall/BUILD.bazel:3:1: Illegal ambiguous match on configurable attribute "deps" in @org_golang_google_grpc//internal/syscall:go_default_library:
@io_bazel_rules_go//go/platform:android
@io_bazel_rules_go//go/platform:darwin
Multiple matches are not allowed unless one is unambiguously more specialized.
INFO: SHA256 (http://jcenter.bintray.com/org/ow2/asm/asm/6.2/asm-6.2.jar) = 917bda888bc543187325d5fbc1034207eed152574ef78df1734ca0aee40b7fc8
ERROR: Analysis of target '//android:publish_snapshot' failed; build aborted:

/private/var/tmp/_bazel_steeve/309ac22c92b055a3e3f78825fe63406b/external/org_golang_google_grpc/internal/syscall/BUILD.bazel:3:1: Illegal ambiguous match on configurable attribute "deps" in @org_golang_google_grpc//internal/syscall:go_default_library:
@io_bazel_rules_go//go/platform:android
@io_bazel_rules_go//go/platform:darwin
Multiple matches are not allowed unless one is unambiguously more specialized.
INFO: Elapsed time: 6.105s
INFO: 0 processes.
FAILED: Build did NOT complete successfully (23 packages loaded, 8805 targets configured)

[steeve]

Note that this happens also on bazel 0.25.3
android_arm

/private/var/tmp/_bazel_steeve/309ac22c92b055a3e3f78825fe63406b/external/org_golang_google_grpc/internal/channelz/BUILD.bazel:3:1: Illegal ambiguous match on configurable attribute "deps" in @org_golang_google_grpc//internal/channelz:go_default_library:
@io_bazel_rules_go//go/platform:darwin
@io_bazel_rules_go//go/platform:android
Multiple matches are not allowed unless one is unambiguously more specialized.

android_arm64:

/private/var/tmp/_bazel_steeve/309ac22c92b055a3e3f78825fe63406b/external/org_golang_google_grpc/internal/channelz/BUILD.bazel:3:1: Illegal ambiguous match on configurable attribute "deps" in @org_golang_google_grpc//internal/channelz:go_default_library:
@io_bazel_rules_go//go/platform:darwin
@io_bazel_rules_go//go/platform:android
Multiple matches are not allowed unless one is unambiguously more specialized.

[steeve]

cc @jin since he made the NDK rules
@jin could it be that the NDK rules suffered from the same bug from bazelbuild/bazel#8522 ?

[steeve]

Hum this is weird, when I constrain on @bazel_tools//platforms:aarch64 instead of @io_bazel_rules_go//go/toolchain:arm64, I get the same issue as android_arm.

[jayconrod]

it seems I can't use the @io_bazel_rules_go//go/toolchain:ios constraint with your patch. I suspect only the os_arch combination exist ?
The reason is we have our own platform that used to apply rules_go constraints

Some explanation about the naming here: //go/platform contains config_setting targets for each GOOS, GOARCH, and GOOS_GOARCH pair. //go/toolchain contains constraint_setting, constraint_value, and platform targets. Ideally platform targets would be in //go/platform, but that package predates Bazel support for platforms and toolchains by quite a bit, and displacing the old config_settings would break builds.

Anyway, you should be able to define a platform like this:

platform(
    name = "custom_ios_arm",
    constraint_values = [
        "@bazel_tools//platforms:ios",
        "@bazel_tools//platforms:arm',
        "@io_bazel_rules_go//go/toolchain:is_darwin",  # needed for @io_bazel_rules_go//platform:darwin to work
        # any other values relevant to your toolchains
    ],
)

I think the Android problems you're experiencing are #2089, which is affecting cross-compilation in general. I'm planning to fix that in another PR before the next release.