Fix two regressions dealing with the kgdb core.

1) kgdb_skipexception and kgdb_post_primary_code are optional
functions that are only required on archs that need special exception
fixups.

2) The kernel address space scope must be set on any probe_kernel_*
function or archs such as ARCH=arm will not allow access to the kernel
memory space.  As an example, it is required to allow the full kernel
address space is when you the kernel debugger to inspect a system
call.
Signed-off-by: NJason Wessel <jason.wessel@windriver.com>
Signed-off-by: NIngo Molnar <mingo@elte.hu>

add probe_kernel_read() and probe_kernel_write().

Uninlined and restricted to kernel range memory only, as suggested
by Linus.
Signed-off-by: NIngo Molnar <mingo@elte.hu>
Reviewed-by: NThomas Gleixner <tglx@linutronix.de>