rs6000/test: Adjust pr104992.c with vect_int_mod [PR106516]

rs6000/test: Adjust pr104992.c with vect_int_mod [PR106516]

As PR106516 shows, we can get unexpected gimple outputs for
function thud on some target which supports modulus operation
for vector int.  This patch introduces one effective target
vect_int_mod for it, then adjusts the test case with it.

	PR testsuite/106516

gcc/testsuite/ChangeLog:

	* gcc.dg/pr104992.c: Adjust with vect_int_mod.
	* lib/target-supports.exp (check_effective_target_vect_int_mod): New
	effective target.