[compiler-rt] [compiler-rt] Allow building builtins.a without a libc (PR #86737)

[Alexander Richardson via llvm-commits]

https://github.com/arichardson approved this pull request.

Thanks, I've definitely done this in the past and we should continue to keep this working. I wonder if we could add a basic "cross compile the builtins for baremetal" GitHub action?

https://github.com/llvm/llvm-project/pull/86737