Auto merge of #51063 - mixi:musl-bootstrap, r=alexcrichton

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 (6d9154a830dd9773fe8a4e34e1fc3dfb1ca6f935) 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