auto merge of #6105 : Aatch/rust/linker-improv, r=pcwalton

Adds two extra flags: `--linker` which takes extra flags to pass to the linker, can be used multiple times and `--print-link-args` which prints out linker arguments. Currently `--print-link-args` needs execution to get past translation to get the `LinkMeta` data.

I haven't done tests or updated any extra documentation yet, so this pull request is currently here for review.

diff --cc src/librustc/driver/driver.rs
Simple merge