Support custom GOARM architecture levels via platform constraints

[LINKIWI]

What type of PR is this?

Feature

What does this PR do? Why is it needed?

This PR introduces support for specifying the ARM architecture level as GOARM via platform constraints, mirroring the implementation in #3251 which accomplished a similar objective for GOAMD64.

The constraint names are taken from here: https://go.dev/wiki/MinimumRequirements#arm

Which issues(s) does this PR fix?

Fixes #1719

Other notes for review

Verification:

WORKSPACE:

local_repository(
    name = "io_bazel_rules_go",
    path = "/path/to/rules_go/fork",
)

load("@io_bazel_rules_go//go:deps.bzl", "go_register_toolchains", "go_rules_dependencies")

go_rules_dependencies()

go_register_toolchains(version = "1.21.6")

BUILD:

load("@io_bazel_rules_go//go:def.bzl", "go_binary", "go_cross_binary", "go_library")

platform(
    name = "goarm_5",
    constraint_values = [
        "@platforms//os:linux",
        "@platforms//cpu:armv7",
        "@io_bazel_rules_go//go/constraints/arm:5",
    ],
)

platform(
    name = "goarm_6",
    constraint_values = [
        "@platforms//os:linux",
        "@platforms//cpu:armv7",
        "@io_bazel_rules_go//go/constraints/arm:6",
    ],
)

platform(
    name = "goarm_7",
    constraint_values = [
        "@platforms//os:linux",
        "@platforms//cpu:armv7",
        "@io_bazel_rules_go//go/constraints/arm:7",
    ],
)

go_library(
    name = "lib",
    srcs = ["main.go"],
    importpath = "sample",
)

go_binary(
    name = "main_host",
    out = "main",
    embed = [":lib"],
)

go_cross_binary(
    name = "main_arm_5",
    platform = ":goarm_5",
    target = ":main_host",
)

go_cross_binary(
    name = "main_arm_6",
    platform = ":goarm_6",
    target = ":main_host",
)

go_cross_binary(
    name = "main_arm_7",
    platform = ":goarm_7",
    target = ":main_host",
)

main.go:

package main

import (
	"fmt"
)

func main() {
	fmt.Println("Hello, world!")
}

Compile steps:

$ bazel build //:main_arm_5
$ bazel build //:main_arm_6
$ bazel build //:main_arm_7

It seems that the Go compiler does not embed ARM attributes in the compiled binaries, so readelf -A <binary> does not produce any output. This would otherwise indicate the ARM architecture level in the Tag_CPU_arch tag.

So instead I copied each of these binaries to an ARM host with the following CPU feature set support:

$ cat /proc/cpuinfo
processor	: 0
model name	: ARMv6-compatible processor rev 7 (v6l)
BogoMIPS	: 697.95
Features	: half thumb fastmult vfp edsp java tls 
CPU implementer	: 0x41
CPU architecture: 7
CPU variant	: 0x0
CPU part	: 0xb76
CPU revision	: 7

Hardware	: BCM2835
Revision	: 100000e
Serial		: 00000000557be544
Model		: Raspberry Pi Model B Rev 2

Results:

$ ./main_arm_5
Hello, world!
$ ./main_arm_6
Hello, world!
$ ./main_arm_7
Illegal instruction