[AArch64] Alter arm_sve.h to be target-based, not preprocessor based.

This patch makes SVE intrinsics more useable by gating them on the
target, not by ifdef preprocessor macros. See #56480. This alters the
SVEEmitter for arm_sve.h to remove the #ifdef guards and instead use
TARGET_BUILTIN with the correct features so that the existing "'func'
needs target feature sve" error will be generated when sve is not
present.

The ArchGuard containing defines in the SVEEmitter are changed to
TargetGuard containing target features. In the arm_neon.h emitter there
are both existing ArchGuard ifdefs mixed with new TargetGuard target
feature guards, so the name is change in the SVE too for consistency.
The few functions that are present in arm_sve.h (as opposed to builtin
aliases) have __attribute__((target("sve"))) added. Some of the tests
needed to be rejigged a little, as well as updating the error message,
as the error now happens at a later point.

Differential Revision: https://reviews.llvm.org/D131064