s390: Allocate gdbarch & tdep at start of gdbarch_init

s390: Allocate gdbarch & tdep at start of gdbarch_init

Moving the allocation of gdbarch_tdep to the start of s390_gdbarch_init
allows us to use its fields for tracking the different features instead of
using separate variables.  To make the code a little nicer move the actual
allocation and initialization to a separate function.  Also move the
allocation of gdbarch to keep the two together.

gdb/ChangeLog:

	* s390-linux-tdep (s390_abi_kind) <ABI_NONE>: New default field.
	(gdbarch_tdep) <have_upper, have_vx>: New fields.
	(s390_gdbarch_tdep_alloc): New function.
	(s390_gdbarch_init): Allocate tdep at start and use its fields
	instead of separate variables.