[SPARC64]: Move signal compat bits to new header file.

Create asm-sparc64/compat_signal.h and stuff things there.

This avoids the "linux/compat.h includes asm/signal.h but
asm/signal.h needs compat_sigset_t which isn't defined yet"
problems introduced recently.

Signed-off-by: David S. Miller <davem@davemloft.net>