Fixed second issue reported in https://bugs.llvm.org/show_bug.cgi?id=41584.

Added synchronization for possible concurrent initialization of mutexes
by multiple threads. The need of synchronization caused by commit r357927
which added the use of mutexes at threads movement to/from common pool
(earlier the mutexes were used only at suspend/resume).

Patch by Johnny Peyton.

Differential Revision: https://reviews.llvm.org/D61995

llvm-svn: 360919