* gdbarch-utils.h, gdbarch-utils.c: New files.

* gdbarch-utils.h, gdbarch-utils.c: New files.
* Makefile.in (SFILES, COMMON_OBS): Update.
(gdbarch_utils_h) Define.
(gdbarch-utils.o): Add dependencies.

* gdbarch.c, gdbarch.sh: Include "gdbarch-utils.h". Fix code
handling default method values.
(startup_gdbarch): Rename default_gdbarch, name misleading.
(breakpoint_from_pc): Default to legacy_breakpoint_from_pc.
(register_name): Default to legacy_register_name.
(call_dummy_words): Default to legacy_call_dummy_words.
(sizeof_call_dummy_words): Default to
legacy_sizeof_call_dummy_words.
(register_convertible): Default to
generic_register_convertible_not.
(breakpoint_from_pc): Default to legacy_breakpoint_from_pc.
(remote_translate_xfer_address): Default to
generic_remote_translate_xfer_address.
(frameless_function_invocation): Default to
generic_frameless_function_invocation_not.