internal/cpu: deduplicate arm64 ISAR parsing code

internal/cpu: deduplicate arm64 ISAR parsing code

Deduplicate code for parsing system registers - this matches what is done
in golang.org/x/sys/cpu.

Change-Id: If3524eb2e361179c68678f8214230d7068fe4c60
Reviewed-on: https://go-review.googlesource.com/c/go/+/422217
Reviewed-by: Meng Zhuo <mzh@golangcn.org>
Reviewed-by: Dmitri Shuralyov <dmitshur@google.com>
Run-TryBot: Joel Sing <joel@sing.id.au>
Reviewed-by: Cherry Mui <cherryyz@google.com>
Reviewed-by: Tobias Klauser <tobias.klauser@gmail.com>
TryBot-Result: Gopher Robot <gobot@golang.org>