rust-common: provide C++ wrappers

For similar reasons to the C and linker wrappers, using rustbuild
requires a C++ wrapper to ensure that the right flags that Yocto needs
are passed to the underlying compiler.

(cherry picked from commit e814ede9d9)

classes/cargo.bbclass

@@ -70,12 +70,15 @@ oe_cargo_build () {
 
 oe_cargo_fix_env () {
 	export CC="${RUST_TARGET_CC}"
+	export CXX="${RUST_TARGET_CXX}"
 	export CFLAGS="${CFLAGS}"
 	export AR="${AR}"
 	export TARGET_CC="${RUST_TARGET_CC}"
+	export TARGET_CXX="${RUST_TARGET_CXX}"
 	export TARGET_CFLAGS="${CFLAGS}"
 	export TARGET_AR="${AR}"
 	export HOST_CC="${RUST_BUILD_CC}"
+	export HOST_CXX="${RUST_BUILD_CXX}"
 	export HOST_CFLAGS="${BUILD_CFLAGS}"
 	export HOST_AR="${BUILD_AR}"
 }