[MIPS] Update untrusted.gypi
[MIPS] Update untrusted.gypi

Add missing block for translating newlib pexe to MIPS nexe.
Use libstdc++ to match the change from r12484, for consistency with the
builds on x86-64, x86-32 and ARM.

BUG= build breaks for irt.nexe and newlib.nexe for MIPS
TEST= build Chromium with NaCl turned on

Review URL: https://codereview.chromium.org/135303009

git-svn-id: 721b910a23eff8a86f00c8fd261a7587cddf18f8@12650 fcba33aa-ac0c-11dd-b9e7-8d5594d729c2

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