Fix red zone clobber in relocated RIP-relative code on SysV x86-64
Remove unused constant
Rename define to be clearer
Bump Stalker footprint to 256 MB

Apparently 128 MB is not enough for a typical Cocoa 64-bit application on
Mac. In the future there should be a background thread adding new slabs
periodically.
Remove Stalker test workaround that's no longer needed