Fix TYPE_SPECIFIC_FIELD for types created via arch_type

Fix TYPE_SPECIFIC_FIELD for types created via arch_type

A type's TYPE_SPECIFIC_FIELD is supposed to be initialized as appropriate
for the type code.  This does happen if the type is created via init_type,
but not if it created via arch_type.

Fixed by extracting the initialization logic into a new set_type_code
routine, which is then called from both places.

gdb/ChangeLog:

	* gdbtypes.c (set_type_code): New function.
	(init_type, arch_type): Use it.

Signed-off-by: Ulrich Weigand <ulrich.weigand@de.ibm.com>