MIPS: Factor o32 specific code into signal_o32.c

The commit ebb5e78cc6 ("MIPS: Initial implementation of a VDSO")
caused building a 64 bit kernel with support for n32 and not o32
to produce a build error:

arch/mips/kernel/signal32.c:415:11: error: ‘vdso_image_o32’ undeclared here (not in a function)
  .vdso  = &vdso_image_o32,

Fix this by moving the o32 specific code into signal_o32.c and
updating the Makefile accordingly.

Signed-off-by: Harvey Hunt <harvey.hunt@imgtec.com>
Cc: Leonid Yegoshin <Leonid.Yegoshin@imgtec.com>
Cc: Alex Smith <alex@alex-smith.me.uk>
Cc: linux-mips@linux-mips.org
Cc: linux-kernel@vger.kernel.org
Patchwork: https://patchwork.linux-mips.org/patch/13690/
Signed-off-by: Ralf Baechle <ralf@linux-mips.org>

arch/mips/include/asm/signal.h

@@ -11,7 +11,7 @@
 
 #include <uapi/asm/signal.h>
 
-#ifdef CONFIG_MIPS32_COMPAT
+#ifdef CONFIG_MIPS32_O32
 extern struct mips_abi mips_abi_32;
 
 #define sig_uses_siginfo(ka, abi)                               \