Having already merged the ppc and ppc64 versions of signal.c, this
patch finishes the job by merging signal.h.  The two versions were
almost identical already.  Notable changes:
	- We use BITS_PER_LONG to correctly size sigset_t
	- Remove some uneeded #includes and struct forward
declarations.  This does mean adding an include to signal_32.c which
relied on the indirect inclusion of sigcontext.h
	- As the ppc64 version, the merged signal.h has prototypes for
do_signal() and do_signal32().  Thus remove extra prototypes from
ppc_ksyms.c which had them directly.

Built and booted on POWER5 LPAR (ARCH=ppc64 and ARCH=powerpc).  Built
for 32-bit powermac (ARCH=ppc and ARCH=powerpc) and Walnut (ARCH=ppc).
Signed-off-by: David Gibson <david@gibson.dropbear.id.au>
Signed-off-by: Paul Mackerras <paulus@samba.org>