arm: MVE: Add C++ polymorphism and fix some more issues

arm: MVE: Add C++ polymorphism and fix some more issues

This patch adds C++ polymorphism for the MVE intrinsics, by using the native C++
polymorphic functions when C++ is used.

It also moves the PRESERVE name macro definitions to the right place so that the
variants without the '__arm_' prefix are not available if we define the PRESERVE
NAMESPACE macro.

This patch further fixes two testisms that were brought to light by C++ testing
added in this patch.

gcc/ChangeLog:
2020-04-07  Andre Vieira  <andre.simoesdiasvieira@arm.com>

	* config/arm/arm_mve.h: Add C++ polymorphism and fix preserve MACROs.

gcc/testsuite/ChangeLog:
2020-04-07  Andre Vieira  <andre.simoesdiasvieira@arm.com>

	* g++.target/arm/mve.exp: New.
	* gcc.target/arm/mve/intrinsics/vcmpneq_n_f16: Fix testism.
	* gcc.target/arm/mve/intrinsics/vcmpneq_n_f32: Likewise.