[tsan] Make tests that use CLOCK_MONOTONIC portable

Several tests rely on CLOCK_MONOTONIC, which doesn't exist on OS X. This patch fixes these tests by either disabling them (in case of cond_version.c which doesn't make sense on OS X), or by porting the test to also work on OS X.

Differential Revision: http://reviews.llvm.org/D14861

llvm-svn: 253658