asm.make replaces source files listed in asm/SOURCES with
a corresponding .c or .S file under firmware/asm/${ARCH}.
This makes it difficult to handle differences within one
architecture, eg. different instruction sets or versions
of the architecture.
Get rid of asm.make, and instead use the preprocessor to
select the architecture-specific sources directly from
asm/SOURCES. This is slightly more verbose but is more
flexible.
Change-Id: Id2a484794d5d951b7ba7b54a552de6960414fa45