Reland "gn build: Fix support for building the builtins for baremetal."

Our support for building for baremetal was conditional on a default
off arg and would have failed to build if you had somehow arranged
to pass the correct --target flag; presumably nobody noticed because
nobody was turning it on. A better approach is to model baremetal
as a separate "OS" called "baremetal" and build it in the same way
as we cross-compile for other targets. That's what this patch does.
I only hooked up the arm64 target but others can be added.

Relanding after fixing Mac build breakage in D123244.

Differential Revision: https://reviews.llvm.org/D122862