[tsan] Add Darwin support for lit tests

This patch enables running lit tests on OS X:
1) Simply enable tests for Darwin (they were restricted to Linux and FreeBSD).
2) Disable using instrumented libcxx (libcxx_tsan) on Darwin.
3) On Darwin, override abort_on_error=0, otherwise all tests would generate crash logs and take much longer to process.

Differential Revision: http://reviews.llvm.org/D14439

llvm-svn: 252309