cmd/link: turn ASLR off for netbsd+race

The race detector can't handle ASLR (adddress space layout randomization).
On some platforms it can re-exec the binary with ASLR off. But not NetBSD.
For NetBSD we have to introduce a special ELF header note that tells
the kernel not to use ASLR.

This works fine for internal linking. For external linking it also works,
but "readelf -n" shows multiple notes in the resulting binary. Maybe the
last one wins? Not sure, but it appears to work.

Change-Id: I5fe6dd861e42a8293f64d0dacb166631ea670fcc
Reviewed-on: https://go-review.googlesource.com/c/go/+/227864


Run-TryBot: Keith Randall <khr@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Dmitry Vyukov <dvyukov@google.com>