Fix headless on buildbot (command line length), speed up further

As far as I can guess, I think it's hitting command line limits.  So let's just send via stdin, it's easy and safe.  I did some experimenting with the build steps so I'm mostly sure this is the problem.

PPGe was taking 55% of the total test execution time in LoadZIM(), which isn't something I think is important to test anyway, so I just disabled it in headless.

After that, it was JitBlockCache::Init() taking like 35%, because MIPSOpcode had a constructor.  Killing that improved perf even more.

So now, `test.py -g` takes about 1.6-1.7 seconds, or 10% what it did when it was mostly Python.

FWIW, it now spends 15% of its time in `FPURegCache::Start()`, 7% in `SymbolMap::GetSymbolInfo()` (the lock_guard), 2.5% in `MIPSGetInstruction()`, and 2% in `GPRRegCache::Start()`.

-[Unknown]