OS X unconditionally sets si_code = TRAP_BRKPT when sending SIGTRAP,
even if it was generated by kill -TRAP and not a breakpoint.
Correct the si_code by looking to see if the PC is after a breakpoint.

For #12906.

Change-Id: I998c2499f7f12b338e607282a325b045f1f4f690
Reviewed-on: https://go-review.googlesource.com/18347Reviewed-by: Ian Lance Taylor <iant@golang.org>