Fix some GC issues in the aarch64 back-end.

Fix some GC issues in the aarch64 back-end.

I got some ICEs in my latest testsing while running the libstdc++ testsuite.
I had noticed the problem was connected to types and had just touched the
builtins code but nothing which could have caused this and I looked for
some types/variables that were not being marked with GTY.

OK? Bootstrapped and tested on aarch64-linux-gnu with no regressions.

gcc/ChangeLog:

	* config/aarch64/aarch64-builtins.c (struct aarch64_simd_type_info):
	Mark with GTY.
	(aarch64_simd_types): Likewise.
	(aarch64_simd_intOI_type_node): Likewise.
	(aarch64_simd_intCI_type_node): Likewise.
	(aarch64_simd_intXI_type_node): Likewise.
	* config/aarch64/aarch64.h (aarch64_fp16_type_node): Likewise.
	(aarch64_fp16_ptr_type_node): Likewise.
	(aarch64_bf16_type_node): Likewise.
	(aarch64_bf16_ptr_type_node): Likewise.