kgdb: support for ARCH=arm

kgdb: support for ARCH=arm

This patch adds the ARCH=arm specific a kgdb backend, originally
written by Deepak Saxena <[email protected]> and George Davis
<[email protected]>.  Geoff Levand <[email protected]>,
Nicolas Pitre, Manish Lachwani, and Jason Wessel have contributed
various fixups here as well.

The KGDB patch makes one change to the core ARM architecture such that
the traps are initialized early for use with the debugger or other
subsystems.

[ [email protected]: small cleanups. ]
[ [email protected]: fixed early_trap_init ]

Signed-off-by: Jason Wessel <[email protected]>
Acked-by: Deepak Saxena <[email protected]>