arm: Fix the failing mve scalar shift execution tests.

arm: Fix the failing mve scalar shift execution tests.

In GCC testsuite the MVE scalar shift execution tests (mve_scalar_shifts[1-4].c) are failings
because of executing them on target hardware which doesn't support MVE instructions. This patch
restricts those tests to execute only on target hardware that support MVE instructions.

2020-06-22  Srinath Parvathaneni  <srinath.parvathaneni@arm.com>

gcc/
	* doc/sourcebuild.texi (arm_v8_1m_mve_fp_ok): Add item.
	(arm_mve_hw): Likewise.

gcc/testsuite/
	* gcc.target/arm/mve/intrinsics/mve_scalar_shifts1.c: Modify.
	* gcc.target/arm/mve/intrinsics/mve_scalar_shifts2.c: Likewise.
	* gcc.target/arm/mve/intrinsics/mve_scalar_shifts3.c: Likewise.
	* gcc.target/arm/mve/intrinsics/mve_scalar_shifts4.c: Likewise.
	* lib/target-supports.exp (check_effective_target_arm_mve_hw): Define.