cmd/link: MACHOPLT is executable

The PLT stubs are executable. Put it together with executable
sections, instead of read-only data sections.

Change-Id: I3818414aa0b87c6968c6c7eccce19b0db7c43193
Reviewed-on: https://go-review.googlesource.com/c/go/+/253018


Run-TryBot: Cherry Zhang <cherryyz@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Than McIntosh <thanm@google.com>