rustc: add --asm-syntax flag

This allows the user to specify which asm syntax is desired for
`--emit=asm`

I also considered an environment variable to let it lurk under the covers a bit more but this seemed most optimal for a generic interface.

I asked on irc and didn't hear anything back so I figured I'd just go ahead and do it and amend if necessary. I was torn between a toplevel option and lurking it in behind `-Z` or similar.

I have some concerns that would hold this back from being merged:

* ~~What does this do on arches that can't intel? (my intuition is that `llc` will barf horribly but I have no idea how rustc will deal with that)~~ Haven't tested on arm (rustc builds are /very/ expensive), but unless anyone objects I don't think this should block.
* Is the toplevel option ok?
* Do I need to add tests for this? It seems like it should have them, but I couldn't find any preexisting. If not I will fumble through them.