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-cross.bb

@@ -10,6 +10,7 @@ INHIBIT_DEFAULT_RUST_DEPS = "1"
 # the bits we need explicitly.
 DEPENDS += "rust-llvm-native"
 DEPENDS += "virtual/${TARGET_PREFIX}gcc virtual/${TARGET_PREFIX}compilerlibs virtual/libc"
+DEPENDS += "rust-native"
 
 PROVIDES = "virtual/${TARGET_PREFIX}rust"
 PN = "rust-cross-${TARGET_ARCH}"
@@ -31,14 +32,18 @@ BUILD_POST_LINK_ARGS_append = " -Wl,-rpath=../../lib"
 # We need the same thing for the calls to the compiler when building the runtime crap
 TARGET_CC_ARCH_append = " --sysroot=${STAGING_DIR_TARGET}"
 
-# cross.bbclass is "helpful" and overrides our do_install. Tell it not to.
-do_install () {
-	rust_do_install
+do_configure () {
 }
 
-# using host-strip on target .so files generated by this recipie causes build errors.
-# for now, disable stripping.
-# A better (but more complex) approach would be to mimic gcc-runtime and build
-# the target.so files in a seperate .bb file.
-INHIBIT_PACKAGE_STRIP = "1"
-INHIBIT_SYSROOT_STRIP = "1"
+do_compile () {
+}
+
+do_install () {
+	mkdir -p ${D}${prefix}/${baselib}/rustlib
+	cp ${WORKDIR}/targets/${TARGET_ARCH}* ${D}${prefix}/${baselib}/rustlib
+}
+
+rust_cross_sysroot_preprocess() {
+    sysroot_stage_dir ${D}${prefix}/${baselib}/rustlib ${SYSROOT_DESTDIR}${prefix}/${baselib}/rustlib
+}
+SYSROOT_PREPROCESS_FUNCS += "rust_cross_sysroot_preprocess"