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-common.bbclass

@@ -7,6 +7,7 @@ FILES_${PN}-dbg += "${rustlibdir}/.debug"
 RUSTLIB = "-L ${STAGING_LIBDIR}/rust"
 RUSTFLAGS += "${RUSTLIB}"
 RUSTLIB_DEP ?= "libstd-rs"
+RUST_TARGET_PATH = "${STAGING_LIBDIR_NATIVE}/rustlib"
 
 # Responsible for taking Yocto triples and converting it to Rust triples