Fix x86_64 check for signal handler.

The check for a signal handler was checking for the instruction after
the syscall. On cuttlefish 64 bit, the instruction was not a nop, so
the check failed, and the signal handler was not working. Only check
the instructions up to the syscall instead.

Bug: 168806886

Test: Ran on x86_64.
Change-Id: I605ed22248748f525939e55d734caa5b08c80bb2