rust-bin: get RUST_TARGET_PATH from rust-common

Since commit 06a138a ("rust: drop custom target finding patch"), rustc
is no longer self-sufficient in finding its target specifications. Any
usage of rustc needs RUST_TARGET_PATH set in the environment.

Move the declaration to rust-common.bbclass and add the export to
rust-bin where rustc is invoked.

classes/rust-bin.bbclass

@@ -56,6 +56,7 @@ do_configure () {
 }
 
 oe_runrustc () {
+	export RUST_TARGET_PATH="${RUST_TARGET_PATH}"
 	bbnote ${RUSTC} ${RUSTC_ARCHFLAGS} ${RUSTC_FLAGS} "$@"
 	"${RUSTC}" ${RUSTC_ARCHFLAGS} ${RUSTC_FLAGS} "$@"
 }