MIPS: Reorder ABI determination ahead of target description loading

Move ABI determination code ahead of target description loading so that
architecture information can be adjusted according to the ABI selected,
and then used in OS dependent register information initialization needed
for target description processing.  No functional change.

	gdb/
	* gdb/mips-tdep.c (mips_gdbarch_init): Reorder ABI determination
	ahead of target description loading.