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

[Jon Chesterfield via llvm-commits]

JonChesterfield wrote:

I'm pretty sure the buildbot break is unrelated to this. The sanitizer source that choked on an arm specific GNU header is unrelated to a cmake test file looking for stdio. I see the arm bot is presently green, going to reland this.

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