ARM: kdgb: use <asm/opcodes.h> for data to be assembled as intruction

The arch_kgdb_breakpoint() function uses an inline assembly directive
to assemble a specific instruction using .word. This means the linker
will not treat is as an instruction, and therefore incorrectly swap
the endian-ness if running BE8.

As noted, this code means that kgdb is really only usable on arm32
kernels, and should be made dependant on not being a thumb2 kernel
until fixed. However this is not something to be added to this patch.

Signed-off-by: Ben Dooks <ben.dooks@codethink.co.uk>
Reviewed-by: Dave Martin <Dave.Martin@arm.com>