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

[Jon Chesterfield via llvm-commits]

JonChesterfield wrote:

I'd like to reopen this PR to track it, but there is no button next to the comment where it should be, and the checkbox option silently does nothing. I don't really want to leave the bot broken, but I also don't want @luporl to miss this as I'd like them to change their buildbot. I think my least-bad option is revert this, hack around it locally and spawn an issue to decrease the chance of me forgetting about the local patch.

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