integrated assembler fixes to support Libav's ARM asm

[Janne Grunau]

Hi,

I'm currently trying to get clang trunk to assemble Libav's ARM asm.
These two patches were what I could fix myself in reasonable time.

The remaining missing directives used in Libav are '.altmacro', '.func'
and '.dn'. I've seen that '.dn' is not supported since the effort to
implement it seemed unreasonable for this fringe feature. I have a Libav
patch to test if it's supported and disable the single use if not. '.func'
is just a debugging aid and can easily be omitted if not supported.

'.altmacro' is more important, I've http://llvm.org/bugs/show_bug.cgi?id=18918
and the wip patch in it. I haven't tested the patch since the conflicts
looked non-trivial. Conflicts could result from manually editing the
`git diff` output of a merge commit.

Janne