compiler-rt missing from rustc-1.37.0-src.tar.gz

[maurer]

In #60981 we switched to detecting compiler-rt's presence in src/llvm-project/compiler-rt and enabling the c feature for compiler-builtins if present, falling back to rust-only otherwise.

In the git checkout, this works fine, however the compiler-rt directory is missing from the rustc-1.37.0-src.tar.gz vendored source tarball. As a result, builds from this tarball fall back to rust-only implementations, resulting in possibly less optimized code.