syscall: move ptrace setup to right before exec

syscall: move ptrace setup to right before exec

This will prevent any ptrace calls from getting trace output from the runtime
itself setting up after fork.

Fixes #21428.

Change-Id: I9d835bd5a8f404394eb6237679f2111a72e5bc17
Reviewed-on: https://go-review.googlesource.com/55811
Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>
Reviewed-by: Avelino <t@avelino.xxx>