package/pkg-cargo.mk: fix building cargo packages on 32bit arm

There are duplicated symbols between rustc and gcc. Specifying
--allow-multiple-definition to the linker as workaround until
rustc is fixed.

rust-lang issue: https://github.com/rust-lang/compiler-builtins/issues/420

Fixes: http://autobuild.buildroot.net/results/53f/53f5598b8e520caaa135fb4923c09da382dab329
Signed-off-by: Moritz Bitsch <moritz@h6t.eu>
Signed-off-by: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>

package/pkg-cargo.mk

@@ -46,6 +46,14 @@ PKG_CARGO_ENV = \
 	CARGO_BUILD_TARGET="$(RUSTC_TARGET_NAME)" \
 	CARGO_TARGET_$(call UPPERCASE,$(RUSTC_TARGET_NAME))_LINKER=$(notdir $(TARGET_CROSS))gcc
 
+#
+# This is a workaround for https://github.com/rust-lang/compiler-builtins/issues/420
+# and should be removed when fixed upstream
+#
+ifeq ($(NORMALIZED_ARCH),arm)
+	PKG_CARGO_ENV += RUSTFLAGS="-Clink-arg=-Wl,--allow-multiple-definition"
+endif
+
 HOST_PKG_CARGO_ENV = \
 	$(PKG_COMMON_CARGO_ENV)