Some extensions to the .section directive for ELF and COFF

[Rafael Espíndola]

> I am currently inclined to go with the unique id option. I will
> implement it Monday morning to
> see what it looks like.

So far so good.

An initial attempt found issues with ARM's unwind info (like Alan
Modra suspect there will be in gas), but they were easy to fix and a
bootstrap on a chromebook is going well now.

The extensions are documented in http://llvm.org/docs/Extensions.html

Thanks for all the feedback!

Cheers,
Rafael