aarch64: Add a test for zeroing <64bits>x2_t structures

aarch64: Add a test for zeroing <64bits>x2_t structures

g:8d6c6fbc5271dde433998c09407b30e2cf195420 improved the code
generated for functions like:

  void test_s8 (int8x8x2_t *ptr) { *ptr = (int8x8x2_t) {}; }

Previously we would load zero from the constant pool, whereas
now we just use "stp xzr, xzr".  This patch adds a test for
this improvement.

gcc/testsuite/
	* gcc.target/aarch64/struct_zero.c: New test.