Fix test assumption that Linux implies glibc.

Summary:
This fixes an regression when using bionic introduced in r345173.

I need to follow up and figure out what exactly is implied by
TEST_HAS_C11_FEATURES and see what the correct configuration is for
bionic (new versions should have everything the tests care about,
versions that predate C11 certainly don't), but this gets the tests
back to the old behavior.

Reviewers: EricWF

Reviewed By: EricWF

Subscribers: mclow.lists, christof, ldionne, libcxx-commits, cfe-commits

Differential Revision: https://reviews.llvm.org/D53956

git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@345900 91177308-0d34-0410-b5e6-96231b3b80d8