[libc++] Allow running the bootstrapping CI build on Darwin

Instead of hard-coding the target for our CI nodes, use the default
compiler triple. Also, allow building compiler-rt for the single
specified triple in case we're running on Darwin (otherwise, the
bootstrapping build complains).

Differential Revision: https://reviews.llvm.org/D113683

GitOrigin-RevId: 3016ceb30d12f8d9d9600d3ec66bf2a5bb3a7d32
1 file changed