testsuite: XFAIL some SLP reduction tests for VLA SVE

testsuite: XFAIL some SLP reduction tests for VLA SVE

For variable-length SVE, we can only use SLP for N scalars of type
T if the number of Ts in a vector is a multiple of N.  For ints
this means that N must be 4 or 2, so this patch XFAILs two tests
for N==8.

The exact limit seems inherently target-specific -- variable-length
vectors with a 256-bit granule would work fine -- so I used aarch64_sve
selectors on the XFAILs.

gcc/testsuite/
	* gcc.dg/vect/slp-reduc-4.c: XFAIL test for SLP vectorization
	for variable-length SVE.
	* gcc.dg/vect/slp-reduc-7.c: Likewise.