Choose stdlib for test suite based on target platform.

When compiling programs for the test suite we currently choose which stdlib to
use based on the host platform, but should be basing this on the target
platform.

Test Plan: ./dotest.py $DOTEST_OPTS -t -p TestThreadExit.py
This test previously failed mac->linux most of the time due to using the mac
host's atomic declaration.

Differential Revision: http://reviews.llvm.org/D9797

llvm-svn: 237466