gas: correct ignoring of C-style number suffixes

First of all the respective original changes didn't deal with just 0
having such a suffix - this needs additional logic outside of
integer_constant(). Further bogus suffixes having more than two L-s
were accepted, while valid suffixes with U following the L(s) weren't.
Finally respective tests were introduced for Sparc only.

Reviewed-by: Neal Frager <neal.frager@amd.com>