message: Document that LaunchProcessPosixSpawn is used on NetBSD
  No functional change.
  Sponsored by <The NetBSD Foundation>
diff: @@ -623,7 +623,7 @@ Error Host::RunShellCommand(const Args &args, const FileSpec &working_dir,
  return error;
  }
  -// LaunchProcessPosixSpawn for Apple, Linux, FreeBSD and other GLIBC
  +// LaunchProcessPosixSpawn for Apple, Linux, FreeBSD, NetBSD and other GLIBC
  // systems
  #if defined(__APPLE__) || defined(__linux__) || defined(__FreeBSD__) || \
  