[MIPS] Fix struct sigcontext for N32 userland

The kernel use 64-bit for sc_regs[0], and both N32/N64 userland
expects it was 64-bit.  But size of 'long' on N32 is actually 32-bit.
So this definition make some confusion.  Use __u32 and __u64 for
N32/N64 sigcontext to get rid of this confusion.

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