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/cargo_util.bbclass

@@ -12,7 +12,7 @@ export CARGO_HOME = "${WORKDIR}/cargo_home"
 BASEDEPENDS_append = " cargo-native"
 
 # Ensure we get the right rust variant
-DEPENDS_append_class-target = " virtual/${TARGET_PREFIX}rust"
+DEPENDS_append_class-target = " virtual/${TARGET_PREFIX}rust ${RUSTLIB_DEP}"
 
 # Cargo only supports in-tree builds at the moment
 B = "${S}"