Don't assume PTHREAD_CREATE_JOINABLE is 0 on all systems

Summary:
Lsan was using PTHREAD_CREATE_JOINABLE/PTHREAD_CREATE_DETACHED
as truthy values, which works on Linux, where the values are 0 and 1,
but this fails on OS X, where the values are 1 and 2.

Set PTHREAD_CREATE_DETACHED to the correct value for a given system.

Reviewers: kcc, glider, kubamracek, alekseyshl

Subscribers: llvm-commits

Differential Revision: https://reviews.llvm.org/D31883

llvm-svn: 300221