untrusted.gypi: Remove support for old gcc newlib toolchain
untrusted.gypi: Remove support for old gcc newlib toolchain

build_nexe.py can also be simplified but I'll leave that
for a subsequent CL.

BUG= https://bugs.chromium.org/p/nativeclient/issues/detail?id=4352

Review URL: https://codereview.chromium.org/1581193006

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