testsuite/arm: Factorize and increase coverage in mve-sub_1.c

Use a template macro to factorize the existing test functions.

This patch also adds a version to check subtraction with __fp16 type.

2021-05-17  Christophe Lyon  <christophe.lyon@linaro.org>

	gcc/testsuite/
	* gcc.target/arm/simd/mve-vsub_1.c: Factorize and add __fp16 test.