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}"

classes/rust-vars.bbclass

@@ -2,3 +2,7 @@ export rustlibdir = "${libdir}/rust"
 FILES_${PN} += "${rustlibdir}/*.so"
 FILES_${PN}-dev += "${rustlibdir}/*.rlib"
 FILES_${PN}-dbg += "${rustlibdir}/.debug"
+
+RUSTLIB = "-L ${STAGING_LIBDIR}/rust"
+RUSTFLAGS += "-C rpath -C crate_hash=${BB_TASKHASH} ${RUSTLIB}"
+RUSTLIB_DEP ?= "libstd-rs"

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