Prevent monitor thread creation when KMP_BLOCKTIME="infinite"

When users sets envirable KMP_BLOCKTIME to "infinite" (the time one busy-waits
at barrieres, etc.), the monitor thread is not useful and can be ignored. This
change prevents the creation of the monitor thread when the users sets
KMP_BLOCKTIME to "infinite".

Differential Revision: http://reviews.llvm.org/D15628

llvm-svn: 256061