Fix GNU strerror_r check for Android.

Summary:
Bionic didn't get a GNU style strerror_r until Android M. Until then
we unconditionally exposed the POSIX one. Expand the check to account
for this.

Reviewers: pirama, AndreyChurbanov, jlpeyton

Reviewed By: jlpeyton

Subscribers: openmp-commits, srhines

Differential Revision: https://reviews.llvm.org/D30056

llvm-svn: 297235