JIT64: try enabling dcbz again

This time, check the address carefully beforehand, since apparently some games
do horrible things like running it on non-RAM addresses, or at the very least
virtual addresses.
Removed the auto setting of SEQUENCE_MS because the synchronised audio code has made it unnecessary.
Merge pull request #904 from FioraAeterna/dcbz

JIT64: try enabling dcbz again
Merge pull request #930 from skidau/openal-seq-ms

Removed the auto setting of SEQUENCE_MS because the synchronised audio c...
x64Emitter: add support for shorter EAX forms of instructions

Should save a few bytes of code size here and there.