ARM: signal: sigreturn_codes should be endian neutral to work in BE8

In case of BE8 kernel data is in BE order whereas code stays in LE
order. Move sigreturn_codes to separate .S file and use proper
assembler mnemonics for these code snippets. In this case compiler
will take care of proper instructions byteswaps for BE8 case.
Change assumes that sufficiently Thumb-capable tools are used to
build kernel.

Problem was discovered during ltp testing of BE system: all rt_sig*
tests failed. Tested against the same tests in both BE and LE modes.

Signed-off-by: Victor Kamensky <victor.kamensky@linaro.org>
Reviewed-by: Dave Martin <Dave.Martin@arm.com>
Signed-off-by: Ben Dooks <ben.dooks@codethink.co.uk>