Re-land [libc] Apply no-builtin everywhere, remove unnecessary flags

This is a reland of D126773 / b2a9ea44.

The removal of `-mllvm -combiner-global-alias-analysis` has landed separately
in D128051 / 7b73f537.

And the removal of `-mllvm --tail-merge-threshold=0` is scheduled for
removal in a subsequent patch.