aarch64: Tighten condition on sve/sel* tests

aarch64: Tighten condition on sve/sel* tests

Noticed while testing on a different machine that the sve/sel_*.c
tests require .variant_pcs support but don't test for it.
.variant_pcs post-dates SVE so there shouldn't be a need to test
for both.

gcc/testsuite/
	* gcc.target/aarch64/sve/sel_1.c: Require aarch64_variant_pcs.
	* gcc.target/aarch64/sve/sel_2.c: Likewise.
	* gcc.target/aarch64/sve/sel_3.c: Likewise.