gdb: support async interrupt (ctrl-c) to break into a running guest

The stub drove continue by calling emu_start synchronously and only read
the socket again once the target stopped on its own, so the bare \x03 break
byte a client sends to pause a running target was never seen. A guest that
free-runs (e.g. an idle/event loop) could not be interrupted at all --
gdb/Ghidra reported 'Cannot execute this command while the target is running'.

Poll the client socket for the break byte from the per-instruction run hook
(dbg_hook), which does run on the emulation thread during emu_start:

- GdbSerialConn.poll_interrupt(): non-blocking select+recv, True on \x03.
- QlGdbUtils.dbg_hook: throttled (every INTR_POLL_INTERVAL insns) check of an
  installed check_interrupt callback; on a break, stop emulation and record it.
- handle_c: reply SIGINT when the stop was an interrupt rather than a
  breakpoint or normal exit.

Verified against a free-running MIPS64 BE guest: \x03 -> S02 in <1ms; the
existing gdb-stub regression test still passes.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

Author: retrocpugeek

qiling/debugger/gdb/utils.py

@@ -35,6 +35,14 @@ def __init__(self, ql: Qiling, entry_point: int, exit_point: int):
         self.interrupted = False
         self._poll_counter = 0
 
+        # async-interrupt support: `check_interrupt` is a callable installed by the
+        # gdb stub that returns True when the client sent a break (ctrl-c / \x03)
+        # while the target was running. `interrupted` records that the last resume
+        # stopped for that reason (rather than a breakpoint or normal exit).
+        self.check_interrupt = None
+        self.interrupted = False
+        self._poll_counter = 0
+
         def __entry_point_hook(ql: Qiling):
             ql.hook_del(ep_hret)
             ql.hook_code(self.dbg_hook)