xen: arm: refactor xchg and cmpxchg into their own headers

xen: arm: refactor xchg and cmpxchg into their own headers

Since these functions are taken from Linux this makes it easier to compare
against the Lihnux cmpxchg.h headers (which were split out from Linux's
system.h a while back).

Since these functions are from Linux the intention is to use Linux coding
style, therefore include a suitable emacs magic block.

For this reason also fix up the indentation in the 32-bit version to use hard
tabs while moving it. The 64-bit version was already correct.

Signed-off-by: Ian Campbell <ian.campbell@citrix.com>
Acked-by: Tim Deegan <tim@xen.org>
Acked-by: Julien Grall <julien.grall@linaro.org>