rustc: filter out empty linker args

This is inspired by a mystifying linker failure when using `pkg-config` to
generate the linker args: `pkg-config` produces output that ends in a
space, thus resulting in an empty linker argument.

Also added some updates to the concerning error messages that helped
spotting this bug.
auto merge of #10749 : Blei/rust/fix-linker-args, r=alexcrichton

This is inspired by a mystifying linker failure when using `pkg-config` to
generate the linker args: `pkg-config` produces output that ends in a
space, thus resulting in an empty linker argument.

Also added some updates to the concerning error messages that helped
spotting this bug.