runtime: add explicit `INT $3` at end of Darwin amd64 sigtramp

The omission of this instruction could confuse the traceback code if a
SIGPROF occurred during a signal handler.  The traceback code would
trace up to sigtramp, but would then get confused because it would see a
PC address that did not appear to be in the function.

Fixes #16453.

Change-Id: I2b3d53e0b272fb01d9c2cb8add22bad879d3eebc
Reviewed-on: https://go-review.googlesource.com/25104


Reviewed-by: Josh Bleecher Snyder <josharian@gmail.com>