[PATCH] Add support for subsections to the ELF assembler. Fixes PR8717.

[Rafael Espíndola]

> Hexagon's gcc emits negative subsection numbers.  The only documented place
> I could find that suggested this is OK was in gcc/config.in,
> HAS_GAS_SUBSECTION_ORDERING.  I looked for some documented range for this
> but could not find any.

I guess we can add support for that, but please check with the gcc
devs if this is intentional and if so why and what is the common usage
(values, how many subsections per file, etc).

Cheers,
Rafael