rust: Ensure buildpaths are handled in debug symbols correctly

To ensure buildpaths aren't encoded in the debug symbol output, ensure
we pass our prefix remapping compiler options in.

(From OE-Core rev: c7a529743560c37973787e4ab2433cb7485afb86)

Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>

meta/recipes-devtools/rust/rust.inc

@@ -177,6 +177,8 @@ rust_runx () {
     unset CXXFLAGS
     unset CPPFLAGS
 
+    export RUSTFLAGS="${RUST_DEBUG_REMAP}"
+
     # Copy the natively built llvm-config into the target so we can run it. Horrible,
     # but works!
     if [ ${RUST_ALTERNATE_EXE_PATH_NATIVE} != ${RUST_ALTERNATE_EXE_PATH} ]; then