sparc64: clean up compat_sigset_t.seta handling

Use compat_sigset_t rather than opencode the array
Drop "switch (_NSIG_WORDS)" as we know this is always 1
Introduce BUILD_BUG_ON() to catch if this changes

As a side-effect of this fix following sparse warnings:
signal32.c:220:60: warning: invalid access past the end of 'seta' (12 8)
signal32.c:220:42: warning: invalid access past the end of 'seta' (8 8)
signal32.c:219:60: warning: invalid access past the end of 'seta' (20 8)
signal32.c:219:42: warning: invalid access past the end of 'seta' (16 8)
signal32.c:218:60: warning: invalid access past the end of 'seta' (28 8)
signal32.c:218:42: warning: invalid access past the end of 'seta' (24 8)
signal32.c:309:68: warning: invalid access past the end of 'seta' (12 8)
signal32.c:309:46: warning: invalid access past the end of 'seta' (8 8)
signal32.c:308:68: warning: invalid access past the end of 'seta' (20 8)
signal32.c:308:46: warning: invalid access past the end of 'seta' (16 8)
signal32.c:307:68: warning: invalid access past the end of 'seta' (28 8)
signal32.c:307:46: warning: invalid access past the end of 'seta' (24 8)

They all pointed to code that was never executed - so no bugs fixed.

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