RISC-V Regression: Fix FAIL of pr65947-8.c for RVV

This test is testing fold_extract_last pattern so it's more reasonable use
vect_fold_extract_last instead of specifying targets.

This is the vect_fold_extract_last property:
proc check_effective_target_vect_fold_extract_last { } {
    return [expr { [check_effective_target_aarch64_sve]
		   || [istarget amdgcn*-*-*]
		   || [check_effective_target_riscv_v] }]
}

include ARM SVE/GCN/RVV.

It perfectly matches what we want and more reasonable, better maintainment.

gcc/testsuite/ChangeLog:

	* gcc.dg/vect/pr65947-8.c: Use vect_fold_extract_last.