runtime: remove CPU capability workarounds for unsupported FreeBSD versions

This CL removes runtime code working around missing ARM processor capability
information in the auxiliary vector in older FreeBSD versions.

As announced in the Go 1.12 release notes Go 1.13 will require FreeBSD 11.2+
or FreeBSD 12.0+. These FreeBSD versions support CPU capability detection
through AT_HWCAP and AT_HWCAP2 values stored in the auxiliary vector.

Updates #27619

Change-Id: I2a457b578d35101a7a5fd56ae9b81b300ad17da4
Reviewed-on: https://go-review.googlesource.com/c/go/+/165799


Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Reviewed-by: Yuval Pavel Zholkover <paulzhol@gmail.com>
Reviewed-by: Tobias Klauser <tobias.klauser@gmail.com>
Run-TryBot: Martin Möhrmann <martisch@uos.de>
TryBot-Result: Gobot Gobot <gobot@golang.org>