Several .s files for ARM had several properties the new assembler will not support.
These include:

- mentioning SP or PC as a hardware register
	These are always pseudo-registers except that in some contexts
	they're not, and it's confusing because the context should not affect
	which register you mean. Change the references to the hardware
	registers to be explicit: R13 for SP, R15 for PC.
- constant creation using assignment
	The files say a=b when they could instead say #define a b.
	There is no reason to have both mechanisms.
- R(0) to refer to R0.
	Some macros use this to a great extent. Again, it's easy just to
	use a #define to rename a register.

Change-Id: I002335ace8e876c5b63c71c2560533eb835346d2
Reviewed-on: https://go-review.googlesource.com/4822Reviewed-by: Dave Cheney <dave@cheney.net>