aarch64: Fix up GC of aarch64_simd_types [PR113270]

aarch64: Fix up GC of aarch64_simd_types [PR113270]

The r14-6524 changes created aarch64-builtins.h header and moved
struct aarch64_simd_type_info definition in there.
Unfortunately, the new header wasn't added to target_gtfiles, so the
trees and const char * pointer elements in the aarch64_simd_types
array aren't marked as GC roots anymore.  That breaks e.g. PCH, when
the array elements then can refer to ggc_freed memory instead of the expected
types, but also any other GC collection could free them and further uses would
not work correctly.

Unfortunately, just adding the new header to target_gtfiles doesn't fix this,
because non-static variable definitions marked with GTY(()) aren't considered
by gengtype, it looks in those cases for an extern GTY(()) declaration, and
there was none - the aarch64-builtins.h header contains an extern declaration
without GTY(()).  Adding GTY(()) to that extern declaration doesn't work, because
then gengtype attempts to emit the aarch64_simd_types GC roots in gtype-desc.cc
but the corresponding header isn't included there.

So, the patch instead adds another extern declaration in aarch64-builtins.cc
right before the actual definition, which makes sure the GC roots are registered
correctly in gt-aarch64-builtins.h (where we want them).

2024-01-09  Jakub Jelinek  <jakub@redhat.com>

	PR target/113270
	* config.gcc (aarch64*-*-*): Add aarch64-builtins.h to target_gtfiles.
	* config/aarch64/aarch64-builtins.cc (aarch64_simd_types): Add extern
	GTY(()) declaration before the definition, drop GTY(()) drom the
	definition.