Use pthreads for thread-local lsan allocator cache on darwin

Summary:
This patch allows us to move away from using __thread on darwin,
which is requiring for building lsan for darwin on ios version 7
and on iossim i386.

Reviewers: kubamracek, kcc

Subscribers: llvm-commits

Differential Revision: https://reviews.llvm.org/D29994

llvm-svn: 298274