Refactor pthread usage of libcxx.

This patch extracts out all the pthread dependencies of libcxx into the
new header __threading_support. The motivation is to make it easy to
re-target libcxx into platforms that do not support pthread.

Original patch from Fulvio Esposito ( - D11781

Applied with tweaks - D19412

Change-Id: I301111f0075de93dd8129416e06babc195aa936b

git-svn-id: 91177308-0d34-0410-b5e6-96231b3b80d8
10 files changed