c6x: remove c6x signal.h

The c6x signal.h includes the asm-generic version and provides
a couple of extern declarations. David Howells pointed out that
the externs needed to be protected by ifdef __KERNEL__. As it
turns out, the externs aren't really needed since the functions
are only called from asm code. So this patch gets rid of the
c6x signal.h and uses just the asm-generic version.

Signed-off-by: Mark Salter <msalter@redhat.com>