Add `--linker` option to pass flags to the linker
Adds '--print-link-args' that outputs linker arguments that would be used
Change flags to -Z print-link-args and --link-args
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.