Auto merge of #50709 - alexcrichton:revert-musl, r=sfackler

Revert #50105 until regression is fixed

Discovered at https://github.com/rust-lang/rust/pull/50105#issuecomment-388630750 it looks like this caused a regression with i686 musl, so let's revert in the meantime while a fix is worked out

diff --cc src/bootstrap/bin/rustc.rs
Simple merge

diff --cc src/bootstrap/builder.rs
Simple merge

diff --cc src/tools/compiletest/src/runtest.rs
Simple merge