Correctly configure USE/env vars for Rust

Although `LLVM_TARGETS` is USE-forced for _llvm-core/llvm_, it is NOT
forced for _dev-lang/rust_.  The latter needs the target architecture
set in order to build the cross-compiled Rust standard library.

crossdev.sh

@@ -26,15 +26,6 @@ crossdev --stable -t "$1"
 
 ln -s /var/db/repos/gentoo/sys-devel/rust-std /var/db/repos/crossdev/cross-${target}/
 
-mkdir -p
-    /etc/portage/env/dev-lang \
-    /etc/portage/package.accept_keywords \
-    /etc/portage/package.use \
-    && :
-printf 'cross-%s/rust-std **\n' "${target}" \
-    > /etc/portage/package.accept-keywords/rust-cross
-echo 'dev-lang/rust rust-src' \
-    > /etc/portage/package.use/rust-src
 case ${target%%-*} in
 aarch64)
     llvm_target=AArch64
@@ -47,9 +38,20 @@ arm*)
     exit 1
     ;;
 esac
-printf '( %s:%s:%s )\n' \
+
+mkdir -p \
+    /etc/portage/env/dev-lang \
+    /etc/portage/package.accept_keywords \
+    /etc/portage/package.use \
+    && :
+printf 'cross-%s/rust-std **\n' "${target}" \
+    > /etc/portage/package.accept_keywords/rust-cross
+printf 'dev-lang/rust rust-src LLVM_TARGETS: %s\n' \
+    "${llvm_target}" \
+    > /etc/portage/package.use/rust-src
+printf 'RUST_CROSS_TARGETS=( %s:%s:%s )\n' \
     "${llvm_target}" \
     "${target}" \
     "${target}" \
     >> /etc/portage/env/dev-lang/rust
-emerge -vbknuUj cross-${target}/rust-std
+emerge -vbknj cross-${target}/rust-std