Add trait object field types to back/abi.rs, and use them

I've added trt_field_vtable, trt_field_box, and trt_field_tydesc, and
inserted them in place of the "magic numbers" used to access trait
object fields through GEPi().
make link_args use spaces as separators

Lots of linking arguments need to be passed as -Wl,--foo so giving the
comma meaning at the rustc layer makes those flags impossible to pass.

Multiple arguments can now be passed from a shell by quoting the
argument: --link-args='-lfoo -Wl,--as-needed'.
auto merge of #6172 : Sodel-the-Vociferous/rust/rm_trt_obj_magic_nums, r=catamorphism,graydon

I don't know how one would write a separate test for this sort of thing. Building the compiler, and `make check` worked, which should mean I didn't screw anything.
auto merge of #6192 : thestinger/rust/link_args, r=catamorphism

Lots of linking arguments need to be passed as -Wl,--foo so giving the
comma meaning at the rustc layer makes those flags impossible to pass.

Multiple arguments can now be passed from a shell by quoting the
argument: --link-args='-lfoo -Wl,--as-needed'.