rust-cross: don't actually build a compiler

Instead of building a full compiler, we can use rust-native to
cross-compile.  All we need is a cross-compiled standard library, which
libstd-rs builds, and a compiler spec file.  rust-cross is now just used
to generate the json spec file for cross-compiling, which is naturally
much faster.

recipes-devtools/rust/rust.inc

@@ -1,6 +1,5 @@
 # ex: sts=4 et sw=4 ts=8
 inherit rust
-inherit rust-installer
 require rust-shared-source.inc
 require rust-snapshot-2016-05-24.inc