cmd/link: turn ASLR off for netbsd+race
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 <8c99c3a9284e493be632950b84cd789d08ed3e9d@golang.org>
TryBot-Result: Gobot Gobot <66cb808b70d30c07676d5e946fee83fd561249e5@golang.org>
Reviewed-by: Dmitry Vyukov <05e8181be3150e47da8fe2e1f18d381c75496ff4@google.com>

CAFxX/go,nawawi/go,cloudflare/go,cloudflare/go,golang/go,akutz/go,nawawi/go,golang/go,cloudflare/go,CAFxX/go,golang/go,nawawi/go,cloudflare/go,akutz/go,CAFxX/go,golang/go,akutz/go,nawawi/go,CAFxX/go,akutz/go,golang/go,golang/go,cloudflare/go,nawawi/go,nawawi/go,akutz/go,cloudflare/go,golang/go,nawawi/go,akutz/go,golang/go,akutz/go,CAFxX/go,cloudflare/go,CAFxX/go,CAFxX/go,CAFxX/go,cloudflare/go,nawawi/go,akutz/go