internal/cpu: remove platform specific prefix from cpu hwcap variables

internal/cpu: remove platform specific prefix from cpu hwcap variables

Go runtime currently only populates hwcap for ppc64 and arm64.
While the interpretation of hwcap is platform specific the hwcap
information is generally available on linux.

Changing the runtime variable name to cpu_hwcap for cpu.hwcap makes it
consistent with the general naming of runtime variables that are linked
to other packages.

Change-Id: I1e1f932a73ed624a219b9298faafbb6355e47ada
Reviewed-on: https://go-review.googlesource.com/94757
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Run-TryBot: Martin Möhrmann <moehrmann@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>