Fix empty mac .toc generation

If a shared library has no exported symbols grep will return exit code
1 meaning no matches, but this should not be considered an error
during toc generation.

Test: external/clang/build.py
Change-Id: If589da38ad8b844fe1aff4738481cebea75cca42

core/config.mk

@@ -225,7 +225,7 @@ endef
 # Commands to generate .toc file from Darwin dynamic library.
 define _gen_toc_command_for_macho
 $(hide) otool -l $(1) | grep LC_ID_DYLIB -A 5 > $(2)
-$(hide) nm -gP $(1) | cut -f1-2 -d" " | grep -v U$$ >> $(2)
+$(hide) nm -gP $(1) | cut -f1-2 -d" " | (grep -v U$$ >> $(2) || true)
 endef
 
 combo_target := HOST_