gcc: Add 'mcf' thread model support from mcfgthread

This patch adds the new thread model `mcf`, which implements mutexes
and condition variables with the mcfgthread library.

Source code for mcfgthread is available at <https://github.com/lhmouse/mcfgthread>.

config/
	* gthr.m4 (GCC_AC_THREAD_HEADER): Add new case for `mcf` thread
	model