Support (almost) arbitrary foreign architectures

[RalfJung]

Since Miri is just an interpreter, there's really no reason that a Linux host shouldn't be able to emulate e.g. a rust lib compiled for a Windows target.

And we are almost there! With #1057 fixed, rust-lang/rust#56443 is no longer a problem -- a check-only libstd build does not need a target toolchain, no object files are created. The only barrier left is rust-lang/backtrace-rs#253, and I just submitted a fix for that. For some targets we also need a small xargo tweak.

With those patches applied, I tried a bunch of things on my Linux host, and they all behave as expected -- pretty amazing. :)

./miri run-debug tests/run-pass/hello.rs --target x86_64-apple-darwin
./miri run-debug tests/run-pass/hello.rs --target i686-apple-darwin
./miri run-debug tests/run-pass/hello.rs --target x86_64-pc-windows-gnu
./miri run-debug tests/run-pass/hello.rs --target x86_64-pc-windows-msvc
./miri run-debug tests/run-pass/hello.rs --target i686-pc-windows-gnu
./miri run-debug tests/run-pass/hello.rs --target i686-pc-windows-msvc

Once the patches all land, we should start testing some of these combinations on CI.

• Test cross-running Linux, macOS and Windows (MSVC) targets on Linux and macOS hosts (Cross-test targets #1249)
• Test cross-running Linux, macOS on Windows hosts (cross-test on a Windows host #1251)
• Test cross-running Windows (GNU) targets (skip copy of windows-gnu libs in check-only mode japaric/xargo#276)

[RalfJung]

The backtrace PR landed. Now it just needs to land in a release so we can bump the crate in rustc. :)

[RalfJung]

The rustc PR to use the fixed backtrace is in the bors queue: rust-lang/rust#69729

When this works, we might even re-land rust-lang/rust#60156 as this time there should be no linking issues... then HashMap would work on macOS even when isolation is enabled. :)

EDIT: But also note rust-lang/rust#70179