testsuite: Skip pr105250.c for powerpc and s390 [PR105266]

This test case pr105250.c is like its related pr105140.c, which
suffers the error with message like "{AltiVec,vector} argument
passed to unprototyped" on powerpc and s390.  So like commits
r12-8025 and r12-8039, this fix is to add the dg-skip-if for
powerpc*-*-* and s390*-*-*.

gcc/testsuite/ChangeLog:

	PR testsuite/105266
	* gcc.dg/pr105250.c: Skip for powerpc*-*-* and s390*-*-*.