[MIPS] Store sign-extend register values for PTRACE_GETREGS

A comment on ptrace_getregs() states "Registers are sign extended to
fill the available space." but it is not true.  Fix code to match the
comment.  Also fix casts on each caller to get rid of some warnings.

Signed-off-by: Atsushi Nemoto <anemo@mba.ocn.ne.jp>
Signed-off-by: Ralf Baechle <ralf@linux-mips.org>