Teach ARM/MC/ELF about gcc compatible reloc output to get past odd linkage

failures with relocations.

The code committed is a first cut at compatibility for emitted relocations in
ELF .o.

Why do this? because existing ARM tools like emitting relocs symbols as
explicit relocations, not as section-offset relocs.

Result is that with these changes,
1) relocs are now substantially identical what to gcc outputs.
2) larger apps (including many spec2k tests) compile, cross-link, and pass

Added reminder fixme to tests for future conversion to .s form.

llvm-svn: 124996