[build] Make sure to link main executable with pthreads

Otherwise it gets linked in by one of the dependencies of shared
libraries which may be too late and we end up with weird crashes in
std::call_once().

Differential Revision: http://reviews.llvm.org/D21478

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@273302 91177308-0d34-0410-b5e6-96231b3b80d8
3 files changed