Allow passing a custom initial stop reason

[daniel5151]

Right now, gdbstub hard-codes the initial stop reason response to T05thread:<thread-id>; (in response to the ? packet).

This can be annoying in gdbstub integrations where the GDB server is only launched in response to some exceptional event (e.g: a guest OS triple-faulting, a process creating a new thread, etc...), where ideally, you'd want to respond with a more appropriate stop reason (e.g: a more appropriate signal number, a new thread response, etc...)

It'd be nice to add some kind of optional API to gdbstub that allows overriding the initial stop reason.