[ARM] 5381/1: unwind: Reorganise the traps.c code

This patch moves code around in the arch/arm/kernel/traps.c file for
easier integration of the stack unwinding support.

Signed-off-by: Catalin Marinas <catalin.marinas@arm.com>
Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>