kgdb, powerpc: arch specific powerpc kgdb support

This patch removes the old kgdb reminants from ARCH=powerpc and
implements the new style arch specific stub for the common kgdb core
interface.

It is possible to have xmon and kgdb in the same kernel, but you
cannot use both at the same time because there is only one set of
debug hooks.

The arch specific kgdb implementation saves the previous state of the
debug hooks and restores them if you unconfigure the kgdb I/O driver.
Kgdb should have no impact on a kernel that has no kgdb I/O driver
configured.

Signed-off-by: Jason Wessel <jason.wessel@windriver.com>
diff --git a/arch/powerpc/Kconfig b/arch/powerpc/Kconfig
index 4c22242..737ebf9 100644
--- a/arch/powerpc/Kconfig
+++ b/arch/powerpc/Kconfig
@@ -112,6 +112,7 @@
 	select HAVE_FTRACE
 	select HAVE_IDE
 	select HAVE_KPROBES
+	select HAVE_ARCH_KGDB
 	select HAVE_KRETPROBES
 	select HAVE_LMB
 	select HAVE_DMA_ATTRS if PPC64