IBM Z: Add GTY marker to builtin data structures

This adds GTY markers to s390_builtin_types, s390_builtin_fn_types,
and s390_builtin_decls. These were missing causing problems in
particular when using builtins after including a precompiled header.

Unfortunately the declaration of these data structures use enum values
from s390-builtins.h.  This file however is not included everywhere
and is rather large.  In order to include it only for the purpose of
gtype-desc.cc we place a preprocessed copy of it in the build
directory and include only this.

This is going to be backported to GCC 12 and 13.

gcc/ChangeLog:

	* config.gcc: Add s390-gen-builtins.h to target_gtfiles.
	* config/s390/s390-builtins.h (s390_builtin_types)
	(s390_builtin_fn_types, s390_builtin_decls): Add GTY marker.
	* config/s390/t-s390 (EXTRA_GTYPE_DEPS): Add s390-gen-builtins.h.
	Add build rule for s390-gen-builtins.h.