Refactor SetCpStatus into two functions for from-GPU and from-CPU mode rather than a boolean parameter.

This shouldn't affect functionality.  I'm not sure if the breakpoint
distinction is actually necessary (my commit messages from the old
dc-netplay last year claim that breakpoints are broken anyway, but I
don't remember why), but I don't actually need to change this part of
the code (yet), so I'll stick with the trimmings change for now.
A tiny restructuring to allow inlining of FifoCommandRunnable.  Probably useless.
Add Flag support to ChunkFile.h
Unify three types of non-FIFO requests to the GPU thread around Common::Event and Common::Flag.

The only possible functionality change is that s_efbAccessRequested and
s_swapRequested are no longer reset at init and shutdown of the OGL
backend (only; this is the only interaction any files other than
MainBase.cpp have with them).  I am fairly certain this was entirely
vestigial.

Possible performance implications: efbAccessReady now uses an Event
rather than spinning, which might be slightly slower, but considering
the slow loop the flags are being checked in from the GPU thread, I
doubt it's noticeable.

Also, this uses sequentially consistent rather than release/acquire
memory order, which might be slightly slower, especially on ARM...
something to improve in Event/Flag, really.