ARM64EC: Implement BeginSimulation

This is used by the kernel (or UNIX side of ntdll in wine) to jump into
x86 code with the given context as is necessary when e.g. returning from
an exception.

Source/Windows/ARM64EC/Module.S

@@ -37,6 +37,15 @@ enter_jit:
   ldr x16, [x17, #0x40] // ChpeV2CpuAreaInfo->EmulatorData[2] - DispatcherLoopTopEnterEC
   br x16 // DispatcherLoopTopEnterEC(RIP:x9, CPUArea:x17)
 
+  // Invoked by KiUserEmulationDispatcher after e.g. an NtContinue to x86 code
+  // Expects a CONTEXT pointer in x0
+.global BeginSimulation
+BeginSimulation:
+  bl "#SyncThreadContext"
+  ldr x17, [x18, #0x1788] // TEB->ChpeV2CpuAreaInfo
+  ldr x16, [x17, #0x48] // ChpeV2CpuAreaInfo->EmulatorData[3] - DispatcherLoopTopEnterECFillSRA
+  br x16 // DispatcherLoopTopEnterECFillSRA(CPUArea:x17)
+
   // Called into by FEXCore
   // Expects the target code address in x9
 .global ExitFunctionEC