rustc: implement -C link-arg

[japaric]

this flag lets you pass a single argument to the linker but can be
used repeatedly. For example, instead of using:

rustc -C link-args='-l bar' (..)

you could write

rustc -C link-arg='-l' -C link-arg='bar' (..)

This new flag can be used with RUSTFLAGS where -C link-args has
problems with "nested" spaces:

RUSTFLAGS='-C link-args="-Tlayout.ld -nostartfiles"'

This passes three arguments to rustc: -Clink-args="-Tlayout.ld and
-nostartfiles" to rustc. That's not what we meant. But this does
what we want:

RUSTFLAGS='-C link-arg=-Tlayout.ld -C link-arg=-nostartfiles`

cc rust-lang/rfcs#1509

r? @alexcrichton
cc @Zoxc

This needs a test. Any suggestion?

[rust-highfive]

Thanks for the pull request, and welcome! The Rust team is excited to review your changes, and you should hear from @alexcrichton (or someone else) soon.

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[japaric]

This needs a test. Any suggestion?

Added a simple test of using -C link-arg twice.

[retep998]

Can -Clink-arg be combined with -Clink-args? And will -Clink-args continue to support only being passed once?

[japaric]

Can -Clink-arg be combined with -Clink-args?

Yes, both sources get merged.

And will -Clink-args continue to support only being passed once?

This PR doesn't touch -Clink-args.

[jethrogb]

Could you add the attribute counterpart #[link_arg] as well?

[japaric]

@jethrogb That I'm not sure about. The docs discourage using #[link_args] and @alexcrichtonsaid:

This is an intentional design decision of #![link_args], and is one of the reasons it's not generally suitable for passing linker arguments. This "feature" has basically always been slated for deprecation/removal.

[alexcrichton]

@bors: r+

Thanks @japaric!

[bors]

📌 Commit 2f71fa7 has been approved by alexcrichton