sparc: Fix asm/signal.h for 32-bit.

Fix a 32-bit sparc regression reported by Robert Reif.

_NSIG_BPW needs to be 32 for 32-bit and 64 for 64-bit

Tested-by: Robert Reif <reif@earthlink.net>
Signed-off-by: David S. Miller <davem@davemloft.net>