aarch64: Remove lazy SIMD builtin initialisation

aarch64: Remove lazy SIMD builtin initialisation

At one time the aarch64 port registered the Advanced SIMD builtins
lazily, when we first encountered a set of target flags that includes
+simd.  These days we always initialise them at start-up, temporarily
forcing a conducive set of flags if necessary.

This patch removes some vestiges of the old way of doing things.

gcc/
	* config/aarch64/aarch64-protos.h
	(aarch64_init_simd_builtins): Remove prototype.
	* config/aarch64/aarch64-builtins.cc
	(aarch64_simd_builtins_initialized_p): Delete.
	(aarch64_init_simd_builtins): Make static.  Remove protection
	against multiple calls.
	* config/aarch64/aarch64-c.cc (aarch64_pragma_target_parse): Remove
	lazy SIMD builtin initialization.
	* config/aarch64/aarch64.cc
	(aarch64_option_valid_attribute_p): Likewise.