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/cargo.bbclass

@@ -24,7 +24,6 @@ export RUST_BACKTRACE = "1"
 
 RUSTFLAGS ??= ""
 CARGO_BUILD_FLAGS = "-v --target ${HOST_SYS} --release"
-RUST_TARGET_PATH = "${STAGING_LIBDIR_NATIVE}/rustlib"
 
 # This is based on the content of CARGO_BUILD_FLAGS and generally will need to
 # change if CARGO_BUILD_FLAGS changes.

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} ${RUSTC_FLAGS} "$@"
 }

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