Cache the value of MterpShouldSwitchInterpreters()

Add field to the Thread object which stores the value instead (negated).
Explicitly update the field when relevant state changes (which is rare).

This speeds up golem interpreter benchmarks by 3.5%
on average with some benchmarks up to 15% faster.

Test: test.py -b -r --interpreter --host
Change-Id: If2df0d3bf9e69ab50c30102b2648e997927c34d8