libitm: Filter out undo writes that overlap with the libitm stack.

	PR libitm/51855
	* config/generic/tls.h (GTM::mask_stack_top): New.
	(GTM::mask_stack_bottom): Declare.
	* config/generic/tls.c (GTM::mask_stack_bottom): New.
	* local.cc (gtm_undolog::rollback): Filter out any updates that
	overlap the libitm stack.  Add current transaction as parameter.
	* libitm_i.h (GTM::gtm_undolog::rollback): Adapt.
	* beginend.cc (GTM::gtm_thread::rollback): Adapt.
	* testsuite/libitm.c/stackundo.c: New test.

From-SVN: r183172