Make CARGO_BUILD_TARGET accept multiple target triples

[madsmtm]

Problem

It is really nice that Cargo supports compiling for multiple platform target triples (not to be confused with the artifacts that Cargo produces). However, it is inconvenient that this capability does not extend to CARGO_BUILD_TARGET.

Proposed Solution

Make CARGO_BUILD_TARGET take multiple values, potentially comma-separated, such that CARGO_BUILD_TARGET=aarch64-apple-darwin,aarch64-apple-ios cargo build would work the same as cargo build --target aarch64-apple-darwin --target aarch64-apple-ios.

Perhaps we could even extend --target to allow comma-separated target triples too?

As a bit of prior art, x.py in rust-lang/rust supports comma-separated target triples.

Notes

This wouldn't work with custom target triple paths that contain commas. How common are these? And would it be okay to break, since custom target json specs are unstable anyhow?

This was also discussed in #10473, but nothing further was done about this yet (from my understanding?)

[epage]

To clarify, this is talking specifically about CARGO_BUILD_TARGET and not build.target which accepts multiple values as of #10473.

(sometimes people refer to env or config to refer to both)

[weihanglo]

To myself I am still hoping -Zadvanced-env instead of a special case.