sparc: unify signal.h

They were almost identical and with the preapration
patch nothing was needed to be added.

The unified version contains a few sparc64 only definitions
but they are kept as is and not protected by ifdef/endif.
The unified version exports a bit more to userspace then the
32 bit version did.
This is not considered fatal.

Signed-off-by: Sam Ravnborg <sam@ravnborg.org>
Signed-off-by: David S. Miller <davem@davemloft.net>