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

This reverts commit a2982a29fdfcfe2904754815c85f630a4dc6d88c.
Buildbot failure that motivated the revert looks unrelated.

GitOrigin-RevId: d2b63ed0ca5c7eb3f8fc029e3a5705cf0ddab438
1 file changed