testsuite: Adjust gcc.dg/vect/slp-21.c for Arm targets

testsuite: Adjust gcc.dg/vect/slp-21.c for Arm targets

On arm* and aarch64* targets, we can vectorise the second of the main
loops using SLP, not just the third.  As the comments say, whether this
is supported depends on a very specific permutation, so it seemed better
to use direct target selectors.

gcc/testsuite/
	* gcc.dg/vect/slp-21.c: Expect 4 SLP instances to be vectorized
	on arm* and aarch64* targets.