allow build-std to be configured per-target in .cargo/config.toml

[fee1-dead]

What does this PR try to resolve?

-Zbuild-std has some issues with per-pkg-target and bindeps, as seen with #10444 and #9451.

At the same time, since these features essentially enable builds where crates with different targets can co-exist, some people may want to use build-std for one target (in my own project: amdgcn-amd-amdhsa, for OS dev people: x86_64-unknown-none) but not for another target where std is already shipped: #8733

This PR overhauls the way build-std interacts with targets and we're able to fix the issues described above by introducing an additional per-target config in .cargo/config.toml for build-std. Note that these configs co-exist with the global -Zbuild-std flag, and has different behavior whether or not the two are used together: if the global flag is enabled, then all targets will attempt to build their std crates, with per-target config as an override. If the global flag is not used, then only targets that are configured via per-target config will try to build std crates.

This supercedes my two previous attempts at resolving the same issue: #10330 and #11969

How to test and review this PR?

Use .cargo/config.toml:

[target.TARGET]
build-std = ['core']
build-std-features = ['default']

I've already included some essential tests in tests/testsuite/standard_lib.rs such as intersection with per-pkg-target and bindeps. Feel free to suggest additional test cases.

I haven't run the cross compile test locally as I don't know how to get an environment for it on NixOS, will (ab)use the CI for this I guess.

Some unresolved questions:

• Should we introduce and use [target.TARGET.unstable] instead?
• Is there a remaining use for the global flags or maybe we can consider removing the existing -Zbuild-std way of usage?
• How important is it that we gate target.TARGET.build-std = 'xxx' when TARGET isn't being built? (thus not parsed in the current state)

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[fee1-dead]

cc @phil-opp if you want to do some testing/review :)