Avoid tight loop in LockFS. Fixes Prince of Persia 2 slowdown during loading screen

src/core/kernel/support/EmuFS.cpp

@@ -125,15 +125,23 @@ uint32_t fs_lock = 0;
 __declspec(naked) void LockFS()
 {
 	__asm {
-		pushfd
-		pushad
-		spinlock :
-		mov eax, 1
-		xchg eax, fs_lock
-		test eax, eax
-		jnz spinlock
-		popad
-		popfd
+		// Backup Registers
+		pushfd
+		pushad
+
+		// Spin until we can aquire the lock
+		spinlock :
+		call SwitchToThread // Give other threads chance to run, prevents hogging entire timeslice waiting for spinlock
+							// We do this here loop because SwitchToThread will overwrite eax, so it cannot go below
+							// It's not worth wasting the extra cycles of pushing/popping eax to the stack around this call
+		mov eax, 1
+		xchg eax, fs_lock
+		test eax, eax
+		jnz spinlock
+
+		// Restore registers and return
+		popad
+		popfd
 		ret
 	}
 }