Block library shutdown until unreaped threads finish spin-waiting

This change fixes possibly invalid access to the internal data structure during
library shutdown.  In a heavily oversubscribed situation, the library shutdown
sequence can reach the point where resources are deallocated while there still
exist threads in their final spinning loop.  The added loop in
__kmp_internal_end() checks if there are such busy-waiting threads and blocks
the shutdown sequence if that is the case. Two versions of kmp_wait_template()
are now used to minimize performance impact.

Patch by Hansang Bae

Differential Revision: https://reviews.llvm.org/D49452

llvm-svn: 337486