Revert r298274: "Use pthreads for thread-local lsan allocator cache on darwin"

This fixes a failure currently present on the upstream linux boxes (and
reproduces for me as well):
http://lab.llvm.org:8011/builders/sanitizer-x86_64-linux/builds/1130/steps/64-bit%20check-asan-dynamic/logs/stdio

llvm-svn: 298382