cmd/dist: avoid lapsing into x86 builds on ARM64 Macs

We use uname -m to decide the GOHOSTARCH default,
and on my ARM64 Mac laptop, uname -m prints x86_64.

uname -a prints:

Darwin p1.local 21.1.0 Darwin Kernel Version 21.1.0: Wed Oct 13 17:33:01 PDT 2021; root:xnu-8019.41.5~1/RELEASE_ARM64_T6000 x86_64

(Note the x86_64 at the end, consistent with uname -m.)

The effect of this is that make.bash builds an x86 toolchain
even when I start with an ARM64 bootstrap toolchain!
Avoid being tricked by looking for RELEASE_ARM64 instead.

Fixes #50643.

Change-Id: I76eded84bde8009d29419d5982bf964a0bf1c8fd
Reviewed-on: https://go-review.googlesource.com/c/go/+/378894


Trust: Russ Cox <rsc@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>