Clean up assembler code to use % prefix for register names

[th-otto]

This is necessary to be able to compile the sources using an
elf toolchain, and might also be necessary in the near future
when using newer assembler versions.

[mikrosk]

From technical point of view this patch is great but I guess the bigger problem will be to reach an agreement whether this is the path freemint/mintlib/gemlib/cflib etc "coding standards" should take (personally I hate those prefixes). Also, there's the problem of backward compatibility (names with "_" vs. without) for older, already compiled libraries.

On the other hand, I'm sure if accepted it will make some people happy, namely the brown guys as now they have to patch all the sources manually exactly like this.

[th-otto]

@mikrosk I don't like that prefixes either, but in the long term we have no choice about it. As already said, not only the elf toolchain maybe affected by this, you will have the same issue also in general, especially in inline asm, to avoid ambiguities between arguments and register names. And this is not only a matter of the used compiler, but also of binutils, namely gas.

Regarding the '_' issue: that patch should be independent of that, it only takes the definition of USER_LABEL_PREFIX (defined by the compiler to either _ or empty) into account. Since gcc 4.6.4 already defines that, there should be no problem, unless you switch to an even older compiler that is configured for elf ;)

[th-otto]

reopened as new #28 with a clean patch