internal/sysinfo: implement CPUName on bsd systems

internal/sysinfo: implement CPUName on bsd systems

sysctl machdep.cpu.brand_string seems to be standard
across the BSDs. There does not seem to be a standard
way to get the CPU frequency.

Change-Id: Ic986d6c81dd54e1b84544317f2a53ce16801319b
Reviewed-on: https://go-review.googlesource.com/c/go/+/520636
Auto-Submit: Russ Cox <rsc@golang.org>
TryBot-Bypass: Russ Cox <rsc@golang.org>
Reviewed-by: Ian Lance Taylor <iant@google.com>
Reviewed-by: Michael Knyszek <mknyszek@google.com>