internal/cpu: remove parentheses from arm64 feature constants

internal/cpu: remove parentheses from arm64 feature constants

The parentheses are not required for the definitions and it brings
the declaration style in line with other architectures feature bits
defined in internal/cpu.

Change-Id: I86cc3812c1488216779e0d1f0e7481687502e592
Reviewed-on: https://go-review.googlesource.com/126775
Run-TryBot: Martin Möhrmann <moehrmann@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Cherry Zhang <cherryyz@google.com>