aarch64: Ignore target pragmas while defining intrinsics

Refactor the switcher classes into two separate classes:

- sve_alignment_switcher takes the alignment switching functionality,
  and is used only for ABI correctness when defining sve structure
  types.
- aarch64_target_switcher takes the rest of the functionality of
  aarch64_simd_switcher and sve_switcher, and gates simd/sve specific
  parts upon the specified feature flags.

Additionally, aarch64_target_switcher now adds dependencies of the
specified flags (which adds +fcma and +bf16 to some intrinsic
declarations), and unsets current_target_pragma.

This last change fixes an internal bug where we would sometimes add a
user specified target pragma (stored in current_target_pragma) on top of
an internally specified target architecture while initialising
intrinsics with `#pragma GCC aarch64 "arm_*.h"`.  As far as I can tell, this
has no visible impact at the moment.  However, the unintended target
feature combinations lead to unwanted behaviour in an under-development
patch.

This also fixes a missing Makefile dependency, which was due to
aarch64-sve-builtins.o incorrectly depending on the undefined $(REG_H).
The correct $(REGS_H) dependency is added to the switcher's new source
location.

gcc/ChangeLog:

	* common/config/aarch64/aarch64-common.cc
	(struct aarch64_extension_info): Add field.
	(aarch64_get_required_features): New.
	* config/aarch64/aarch64-builtins.cc
	(aarch64_simd_switcher::aarch64_simd_switcher): Rename to...
	(aarch64_target_switcher::aarch64_target_switcher): ...this,
	and extend to handle sve, nosimd and target pragmas.
	(aarch64_simd_switcher::~aarch64_simd_switcher): Rename to...
	(aarch64_target_switcher::~aarch64_target_switcher): ...this,
	and extend to handle sve, nosimd and target pragmas.
	(handle_arm_acle_h): Use aarch64_target_switcher.
	(handle_arm_neon_h): Rename switcher and pass explicit flags.
	(aarch64_general_init_builtins): Ditto.
	* config/aarch64/aarch64-protos.h
	(class aarch64_simd_switcher): Rename to...
	(class aarch64_target_switcher): ...this, and add new members.
	(aarch64_get_required_features): New prototype.
	* config/aarch64/aarch64-sve-builtins.cc
	(sve_switcher::sve_switcher): Delete
	(sve_switcher::~sve_switcher): Delete
	(sve_alignment_switcher::sve_alignment_switcher): New
	(sve_alignment_switcher::~sve_alignment_switcher): New
	(register_builtin_types): Use alignment switcher
	(init_builtins): Rename switcher.
	(handle_arm_neon_sve_bridge_h): Ditto.
	(handle_arm_sme_h): Ditto.
	(handle_arm_sve_h): Ditto, and use alignment switcher.
	* config/aarch64/aarch64-sve-builtins.h
	(class sve_switcher): Delete.
	(class sme_switcher): Delete.
	(class sve_alignment_switcher): New.
	* config/aarch64/t-aarch64 (aarch64-builtins.o): Add $(REGS_H).
	(aarch64-sve-builtins.o): Remove $(REG_H).

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