Cleanup: Reduce cpu specific #ifdefs in common masm code

Cleanup: Reduce cpu specific #ifdefs in common masm code

Centralize the callee save register handling as well as potentially cpu
specific stack frame enter/leave code (like ARM doesn't automatically save the
link register) inside bigger cpu specific #ifdefs instead of sprinkling them
throughout the code. That should make it easier in the future to port to
new calling conventions and architectures.

Change-Id: I92fed7cc3d0f7eb4da86843b7ad59581a64f635f
Reviewed-by: Lars Knoll <lars.knoll@digia.com>