Make the way to get intel-formatted assembly from rustc more discoverable

[scottmcm]

As far as I can tell, nowhere in rustc --help does it say how to control the ASM syntax. I only found it by finding this random gist: https://gist.github.com/bluss/5a088d3f420d12406689439e2940d731

Brainstorming ways to make it more discoverable:

• add an --emit=asm-intel option
• document -C "llvm-args=-x86-asm-syntax=intel" in the help somewhere
• ???

[nagisa]

add an --emit=asm-intel option

can't work because the option is x86-specific.

[hanna-kruppe]

We could make that parameter value only valid for x86 targets right? Not very elegant, but since this is a target-specific concern, there's no way to avoid having something target specific in the CLI (either the availability of an option, or the validity of some value).

I'm not a fan of --emit asm-intel though, because different emit types should be able to exist side by side, but both --emit asm and --emit asm-intel would naturally want to output foo.s. Perhaps a codegen or -Z option that changes the flavor of --emit asm? We already have -Zasm-comments, for comparison.

[TommasoBianchi]

I agree with @rkruppe, the --emit flag is intended to be used with possibly multiple arguments and thus it does not make much sense to add an asm-intel option for it. I think that adding a -Zasm-intel option is probably the best idea, which should work by just passing the -x86-asm-syntax=intel down to LLVM.

[trondsg]

but both --emit asm and --emit asm-intel would naturally want to output foo.s

No, because the extension normally used for intel assembly is .asm, not .s.