aarch64: Reinstate some uses of CONSTEXPR

In 9482a5e4 I'd replaced uses
of CONSTEXPR with direct uses of constexpr.  However, it turns
out that we still have CONSTEXPR for a reason: GCC 4.8 doesn't
implement constexpr properly, and for example rejects things like:

  extern const int x;
  constexpr int x = 1;

This patch partially reverts the previous one.  To make things
more complicated, there are still some things that need to be
constexpr rather than CONSTEXPR, since they are used to initialise
scalar constants.  The patch therefore doesn't change anything
in aarch64-feature-deps.h.

gcc/
	* config/aarch64/aarch64-protos.h: Replace constexpr with
	CONSTEXPR.
	* config/aarch64/aarch64-sve-builtins-base.cc: Likewise.
	* config/aarch64/aarch64-sve-builtins-functions.h: Likewise.
	* config/aarch64/aarch64-sve-builtins-shapes.cc: Likewise.
	* config/aarch64/aarch64-sve-builtins-sve2.cc: Likewise.
	* config/aarch64/aarch64-sve-builtins.cc: Likewise.
	* config/aarch64/aarch64.cc: Likewise.
	* config/aarch64/driver-aarch64.cc: Likewise