Modify Makefile in cros_boot_mode package to use C++ linker.

Build libcros_boot_mode.so with C++ linker rather than the C
linker, since it contains C++ source.

BUG=chromium-os:38770
TEST=Emerged package with new Makefile.

Change-Id: I0154dffb4b3abdc3e354bc235fd838a1b94a0e15
Reviewed-on: https://gerrit.chromium.org/gerrit/43790
Reviewed-by: David James <davidjames@chromium.org>
Reviewed-by: Mike Frysinger <vapier@chromium.org>
Commit-Queue: Caroline Tice <cmtice@chromium.org>
Tested-by: Caroline Tice <cmtice@chromium.org>
diff --git a/Makefile b/Makefile
index 72fa1be..0fd2676 100644
--- a/Makefile
+++ b/Makefile
@@ -22,7 +22,7 @@
 RM_ON_CLEAN += $(OUT)cros_boot_mode
 
 $(OUT)libcros_boot_mode.so: $(filter-out %_main.o,$(BOOT_MODE_OBJECTS))
-	$(call cc_library)
+	$(call cxx_library)
 all: $(OUT)libcros_boot_mode.so
 RM_ON_CLEAN += $(OUT)libcros_boot_mode.so