Jitarm64: Implements spin-loop futex for JIT blocks

[Sonicadvance1]

Needs #3339 merged first.

This will ensure that multiple concurrent SIGBUS handlers in the same
code block doesn't modify the same code.

[bylaws]

I feel like FutexSpinWait probably isn't the best name for this, I haven't see futex used to refer to anything but the Linux syscall, which this explicitly doesn't use

[Sonicadvance1]

I feel like FutexSpinWait probably isn't the best name for this, I haven't see futex used to refer to anything but the Linux syscall, which this explicitly doesn't use

Renamed to SpinWaitLock

[alyssarosenzweig]

There's nothing appropriate in the stl? or provided by Linux? Why do we need to roll our own?

[Sonicadvance1]

There's nothing appropriate in the stl? or provided by Linux? Why do we need to roll our own?

Nothing in the STL, libc, or Linux similar to a spin-loop.
Closest things end up being std::condition_variable or pthread_cond_t, or a kernel futex. All of these are too heavy for what is getting accomplished here, which is a spin-loop/spin-lock.