runtime: remove __go_ptrace on AIX

AIX ptrace syscalls doesn't have the same semantic than the glibc one.
The syscall package is already handling it correctly so disable the new
__go_ptrace C function for AIX.

Change-Id: I5a2abe0dceaf2726ec0f6ad240869a630ab6727b
Reviewed-on: https://go-review.googlesource.com/c/gofrontend/+/256777
Trust: Than McIntosh <thanm@google.com>
Reviewed-by: Ian Lance Taylor <iant@golang.org>