untrusted.gypi: Add support for ARM builds using nacl-clang
untrusted.gypi: Add support for ARM builds using nacl-clang

Extra arguments we were passing to nacl-gcc are not
supported by nacl-clang: -Wno-unused-local-typedefs
-Wno-psabi

BUG=454962
TEST=local testing with qemu

Review URL: https://codereview.chromium.org/931363002

sbc100/native_client,sbc100/native_client,sbc100/native_client,sbc100/native_client,sbc100/native_client,sbc100/native_client