spu: Use ptid from regcache instead of inferior_ptid

spu: Use ptid from regcache instead of inferior_ptid

The implementations of to_fetch_registers/to_store_registers in the spu
code use some functions that rely on inferior_ptid.  It's simpler for
now to set/restore inferior_ptid.

gdb/ChangeLog:

	* spu-linux-nat.c (spu_fetch_inferior_registers,
	spu_store_inferior_registers): Use ptid from regcache, set and
	restore inferior_ptid.
	* spu-multiarch.c (spu_fetch_registers, spu_store_registers):
	Likewise.