[bug#28433] [WIP] Rust update

[Danny Milosavljevic]

Hi Ludo,

> Yes, I think -m32 is a no-op in this case.

Yep.  I just got rustc to build a bootstrap executable (which fails when 
executing it [*], but hey, progress) by making a union-build with all the i686 
libs.

Feels kinda icky, but it seems that propagated-inputs don't remember the 
#:system it was for, so for example if A propagates B and A specifies system 
i686, and X specifies system x86_64 and X depends on A, then X's B will be 
x86_64.  Weird but understandable I guess...

Details:

A:
  propagated-inputs
    B
  arguments system i686

X:
  native-inputs
    A
  arguments system x86_64

Now X's B is x86_64...

The workaround is this:

A:
  arguments system i686
  native-inputs
    B
  union-build out/lib from B (etc)

X:
  native-inputs
    A
  arguments system x86_64

That works.

In our specific case A = "rust-bootstrap", X = "rustc", B = "glibc", "gcc:lib" 
etc.

[*] Tries to load libgcc_s.so.1 from the glibc store directory.  WTF?