Skip setting nnacl_tooldir for MIPS
Skip setting nnacl_tooldir for MIPS

MIPS does not have support for glibc, so skip setting nnacl_tooldir if arch
equals mips32.

BUG= run_run_py_test fails on MIPS
TEST= ./scons platform=mips32 run_run_py_test
NOTRY=true

Review URL: https://codereview.chromium.org/1138773004

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