testsuite: XFAIL some Wstringop-overflow tests ...

... for targets that support vectorization of 2-byte char stores
with unaligned address at plain O2.

2021-12-31  Uroš Bizjak  <ubizjak@gmail.com>

gcc/testsuite/ChangeLog:

	* lib/target-supports.exp (check_vect_slp_store_usage):
	Handle TEST_V2QI_2.
	(check_effective_target_vect_slp_v2qi_store_unalign): New procedure.
	* c-c++-common/Wstringop-overflow-2.c: XFAIL specific tests for
	vect_slp_v2qi_store_unalign targets.
	* g++.dg/warn/Wstringop-overflow-3.C: Ditto.
	* gcc.dg/Wstringop-overflow-28.c: Ditto.
	* gcc.dg/Wstringop-overflow-68.c: Ditto.
	* gcc.dg/Wstringop-overflow-75.c: Ditto.
	* gcc.dg/Wstringop-overflow-76.c: Ditto.