asan: Fix missing FakeStack flag cleanup

asan: Fix missing FakeStack flag cleanup

The FakeStack flag is not zeroed out when can_store_by_pieces()
returns false.  Over time, this causes FakeStack::Allocate() to perform
the maximum number of loop iterations, significantly slowing down the
instrumented program.

Link: https://inbox.sourceware.org/gcc-patches/20250109001702.154685-1-iii@linux.ibm.com/
Signed-off-by: Ilya Leoshkevich <iii@linux.ibm.com>
gcc/ChangeLog:

	* asan.cc (asan_emit_stack_protection): Always zero the flag
	unless it is cleared by the __asan_stack_free_N() libcall.