m68k: Switch to generic siginfo layout

This patch switches m68k to generic siginfo layout.  The custom layout
of m68k's `struct siginfo' had several issues due to not considering
aliasing of members in the union, e.g., _uid32 was at different offsets
in ._kill, ._rt and ._sigchld.

Signed-off-by: Maxim Kuvyrkov <maxim@codesourcery.com>
Signed-off-by: Geert Uytterhoeven <geert@linux-m68k.org>