[clang] Temporarily disable Darwin test for linking against libc++ on non-darwin systems (#170912)
Disable the test added in #170303, which breaks bots that don't use ld
as their linker. This is a temporary and narrow disablement of the test
until we can make it more general again, to get the bots green.
Co-authored-by: Louis Dionne <ldionne.2@gmail.com>