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.

classes/rust.bbclass

@@ -2,11 +2,8 @@ inherit rust-vars
 
 RUSTC = "rustc"
 
-# FIXME: --sysroot might be needed
-RUSTFLAGS += "-C rpath -C crate_hash=${BB_TASKHASH}"
 RUSTC_ARCHFLAGS += "--target=${TARGET_SYS} ${RUSTFLAGS}"
 
-RUSTLIB_DEP ?= "rustlib"
 def rust_base_dep(d):
     # Taken from meta/classes/base.bbclass `base_dep_prepend` and modified to
     # use rust instead of gcc