This is the first piece of the wasm debugging prototype (besides the changes to
add/remove breakpoints in WasmModuleObject made with
https://chromium.googlesource.com/v8/v8.git/+/e699f39caed9a23f8e20bd3a0386a3236e272737).

This changelist adds the infrastructure for a GDB-remote stub that will be used
to manage debugging sessions via the gdb-remote protocol.
It enables the creation and termination of debugging sessions over TCP
connections that are managed in a separate thread.
The logic to actually send, receive and decode GDB-remote packets will be part
of a future changelist.

Build with: v8_enable_wasm_gdb_remote_debugging = true
Run with:
  --wasm-gdb-remote                  Enables Wasm debugging with LLDB
                                     (default: false)
  --wasm-gdb-remote-port             TCP port to be used for debugging
                                     (default: 8765)
  --wasm-pause-waiting-for-debugger  Pauses the execution of Wasm code waiting
                                     for a debugger (default: false)
  --trace-wasm-gdb-remote            Enables tracing of Gdb-remote packets
                                     (default: false)

Note that most of this code is "borrowed" from the code of the Chromium NaCL
GDB-remote stub (located in Chromium in src\native_client\src\trusted\debug_stub).

Implementation details:
- class GdbServer acts as a singleton manager for the gdb-remote stub. It is
  instantiated as soon as the first Wasm module is loaded in the Wasm engine.
- class GdbServerThread spawns the worker thread for the TCP connection.
- class Transport manages the socket connection, in a portable way.
- class Session represents a remote debugging session.
- class Target represents a debugging target and it’s the place where the
  debugging packets will be processed and will implement the logic to debug
  a Wasm engine.

Bug: chromium:1010467
Change-Id: Ib2324e5901f5ae1d855b96b99ef0995d407322b6
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/1923407Reviewed-by: Clemens Backes <clemensb@chromium.org>
Reviewed-by: Benedikt Meurer <bmeurer@chromium.org>
Reviewed-by: Michael Achenbach <machenbach@chromium.org>
Commit-Queue: Paolo Severini <paolosev@microsoft.com>
Cr-Commit-Position: refs/heads/master@{#66379}