[aarch64] Use wzr/xzr for assigning 0 to vector element.

gcc/ChangeLog:
	* config/aarch64/aarch64-simd.md (aarch64_simd_vec_set_zero<mode>):
	New pattern.

gcc/testsuite/ChangeLog:
	* gcc.target/aarch64/vec-set-zero.c: New test.