internal/cpu: make arm64 capability bits naming less verbose

internal/cpu: make arm64 capability bits naming less verbose

This makes the constant names less verbose and aligns them more
with the Linux kernel which uses HWCAP_XXX for the constant names.

Change-Id: Ia7d079b59b57978adc045945951eaa1d99b41fac
Reviewed-on: https://go-review.googlesource.com/91738
Run-TryBot: Martin Möhrmann <moehrmann@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>