internal/cpu: unify HWCap/HWCap2 comments

HWCap and HWCap2 are no longer linknamed into package runtime. Also,
merge two sentences both starting with "These are..." and don't mention
any file name where archauxv is defined, as it become outdated if
support for a new $GOOS/$GOARCH combination is added. This is e.g.
already the case for arm64, where archauxv is also defined for
freebsd/arm64.

Change-Id: I9314a66633736b12e777869a832d8b79d442a6f8
Reviewed-on: https://go-review.googlesource.com/c/go/+/228057


Reviewed-by: Ian Lance Taylor <iant@golang.org>