gn build: unbreak mac (and maybe win) after r351258, r351277

The check-hwasan build files assert that current_os == "linux" || current_os ==
"android", so pull it in only there.

ar is unused on mac, so don't set it in the stage2 toolchain. (It'd be nicer to
use llvm-libtool on mac instead of host libtool, but llvm-libtool doesn't seem
to understand the -no_warning_for_no_symbols flag.)

Differential Revision: https://reviews.llvm.org/D56898

llvm-svn: 351519