don't assume the length of a syscall insn in Task::is_entering_traced_syscall

By happy circumstance, the bare '+ 2' here works for supported architectures, but we have better ways to describe the length of a syscall insn now.

The first time I tested this change on my VM, I saw a few `checkpoint_` failures, but subsequent runs showed no failures.  The `gdb_rr.log` files from those failures looked truncated, though, so it's possible there was some pyexpect problem or similar.