Wire the generated ExecInstruction dispatcher into the interpreter's hot loop

Adds Core/MIPS/InterpreterDispatch.cpp, the checked-in output of
GenerateInterpreterDispatch() (see the previous commit), and hooks it
into RunUntilFast() in MIPSTables.cpp in place of the old
MIPSGetInstruction()-based table walk + indirect call through
instr->interpret. The checked-with-breakpoints/memchecks path
(RunUntilWithChecks) is untouched for now, since it inspects
MIPSInstruction flags directly and correctness there matters most.

Also fixes a real crash in headless.cpp found while testing this:
cmdLineOptions.gpuBackend.value() would throw when unset (e.g. with
--graphics=software), now uses value_or().

Verified with `test.py -g --cpu=interpreter --graphics=software`:
313/314 pass; the one failure (cpu/fpu/fpu) is a pre-existing
interpreter-vs-JIT denormal (flush-to-zero) difference, confirmed to
fail identically with the old table-walking dispatch, so unrelated to
this change.

Adds Tools/update-dispatcher.py to regenerate InterpreterDispatch.cpp
from a built PPSSPPHeadless binary whenever the MIPSTables.cpp tables
change.

Co-Authored-By: Claude Sonnet 5 <[email protected]>
Claude-Session: https://claude.ai/code/session_019SKhm9wKEQzRUsx9mTrrtQ

headless/Headless.cpp

@@ -716,7 +716,7 @@ int main(int argc, const char* argv[]) {
 		return 1;
 #else
 		// TODO: Will we need a larger window for higher resolutions? Well, not if we use buffered rendering.
-		window = CreateHiddenWindow(480, 272, cmdLineOptions.gpuBackend.value(), &windowDesc);
+		window = CreateHiddenWindow(480, 272, cmdLineOptions.gpuBackend.value_or(GPUBackend::OPENGL), &windowDesc);
 		if (!windowDesc.Valid()) {
 			fprintf(stderr, "Failed to create a window for graphics context");
 			return 1;