The build system compiles crt0.S many times to produce independent
object files by defining different -DLxxx symbols for each. This
complicates the build, and has led to some bugs in the source and
build (see the previous cleanup commits in here). The crt_bss.S
file has also been duplicated with crt0.S leading to confusion as
to which one is the "real" one. The only reason to keep them in
one file is to make management of the section numbers when calling
START_CRT_FUNC a little easier, but that seems trivial to check
with a quick grep across all the files.
Considering how complicated the build is now, and the bugs we've
had as a result, split the crt0.S file up into multiple source
files so we don't have to compile it multiple times. This will
also simplify merging it into the top-level Makefile.
