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-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"