syscall: remove ptrace syscall on ppc64

ptrace is available only for 32 bits programs.

Change-Id: Ie0c81b799f5f2c4aae78cd06a34709460529d94f
Reviewed-on: https://go-review.googlesource.com/c/gofrontend/+/252558
Trust: Than McIntosh <thanm@google.com>
Reviewed-by: Ian Lance Taylor <iant@golang.org>