MIPS: uapi/asm/siginfo.h: Fix GCC 4.1.2 compilation

MIPS: uapi/asm/siginfo.h: Fix GCC 4.1.2 compilation

It wasn't until GCC 4.3 I believe that the __SIZEOF_*__ predefined macros
were added.  The change below switches <uapi/asm/siginfo.h> to the
_MIPS_SZLONG macro so that compilation with e.g. GCC 4.1.2 succeeds.
This is a user API header so I think this is even more important, for
older userland support.  The change adds an unsuccessful default too, to
catch any compiler configuration oddities.

Signed-off-by: Maciej W. Rozycki <macro@linux-mips.org>
Cc: linux-mips@linux-mips.org
Patchwork: https://patchwork.linux-mips.org/patch/5630/
Signed-off-by: Ralf Baechle <ralf@linux-mips.org>