Fix building rustc on and for musl hosts.
This fixes all problems I had when trying to compile rustc on a musl-based distribution (with `crt-static = false` in `config.toml`).
This is a fixed version of what ended up being #50105, making it possible to compile rustc on musl targets.
The differences to the old (now merged and subsequently reverted) pull request are:
- The commit (6d9154a830) that caused the regression for which the original commits were reverted in #50709 is left out. This means the corresponding bug #36710 is still not fixed with `+crt-static`.
- The test for issue 36710 is skipped for musl targets (until the issue is properly fixed).
- Building cargo-vendor if `crt-static = false` is needed was broken (cargo-vendor links to some shared libraries if they exist on the system and this produces broken binaries with `+crt-static`)
CC @alexcrichton
The codegen crate contains the code to convert from MIR into LLVM IR,
and then from LLVM IR into machine code. In general it contains code
that runs towards the end of the compilation process.
For more information about how codegen works, see the rustc guide.
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