Stop execution when performing HIO request

Since the HIO request is basically treated like a syscall, we need to
stop emulation while waiting for the GDB client to reply with the
result. This ensures any memory queries etc. that GDB makes to fulfill
the HIO request are accessing memory as it was at the time of the
request, instead of afterwards.

src/core/gdbstub/gdbstub.h

@@ -90,6 +90,8 @@ bool CheckBreakpoint(VAddr addr, GDBStub::BreakpointType type);
 // If set to true, the CPU will halt at the beginning of the next CPU loop.
 bool GetCpuHaltFlag();
 
+void SetCpuHaltFlag(bool halt);
+
 // If set to true and the CPU is halted, the CPU will step one instruction.
 bool GetCpuStepFlag();