Initial patch regarding the Subzero MIPS32 sandboxing stage.
At the moment, the results of the crosstests with vector tests
disabled are as follows:

ASM mode:
19 passing / 5 failing
test_bitmanip: O2
test_calling_conv: Om1, O2
test_sync_atomic: Om1, O2

ELF mode:
15 passing / 9 failing
test_bitmanip: O2
test_calling_conv: Om1, O2
test_global: Om1, O2
test_stacksave: Om1, O2
test_sync_atomic: Om1, O2

R=stichnot@chromium.org

Review URL: https://codereview.chromium.org/2482123002 .

Patch from Stefan Maksimovic <makdstefan@gmail.com>.