s390: testsuite: Xfail forwprop-4{0,1}.c

s390: testsuite: Xfail forwprop-4{0,1}.c

The tests fail on s390 since can_vec_perm_const_p fails and therefore
the bit insert/ref survive which r14-3381-g27de9aa152141e aims for.
Strictly speaking, the tests only fail in case the target supports
vectors, i.e., for targets prior z13 or in case of -mesa the emulated
vector operations are optimized out.

Set to xfail and tracked by PR114802.

gcc/testsuite/ChangeLog:

	* gcc.dg/tree-ssa/forwprop-40.c: Xfail for s390.
	* gcc.dg/tree-ssa/forwprop-41.c: Xfail for s390.
	* lib/target-supports.exp: Add target check s390_mvx.