[mips] Enable IAS by default for 32-bit MIPS targets (O32).
The MIPS IAS can now pass 'ninja check-all', recurse, build a bootable linux
kernel, and pass a variety of LNT testing.
Unfortunately we can't enable it by default for 64-bit targets yet since the N32
ABI is still very buggy and this also means we can't enable it for N64 either
because we can't distinguish between N32 and N64 in the relevant code.