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

[Leandro Lupori via llvm-commits]

luporl wrote:

This patch broke our clang-armv8-lld-2stage buildbot: https://lab.llvm.org/buildbot/#/builders/178/builds/7117.

Can you please take a look at it? Let me know if you need any help to investigate it.

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