rustc: add --asm-syntax flag

[richo]

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.

[rust-highfive]

Warning

• These commits modify unsafe code. Please review it carefully!

[richo]

Any feedback on this? It'd be nice to have it in mainline to avoid having to fiddle so much at my end.

[thestinger]

-C llvm-args=-x86-asm-syntax=intel does allow doing this already. It might make sense to add a way to do it directly via a rustc switch to make it easier and more discoverable but it's not required for the functionality.

[richo]

Oh wow, that is lifechanging.

I'll find somewhere appropriate to put that in the docs. Thanks!