internal/cpu: use anonymous struct for CPU feature vars

Like in x/sys/cpu, use anonymous structs to declare the CPU feature vars
instead of defining single-use types. Also, order the vars
alphabetically.

Change-Id: Iedd3ca51916e3cbb852d2aeed18b3a4c6613e778
Reviewed-on: https://go-review.googlesource.com/c/go/+/221757


Reviewed-by: Ian Lance Taylor <iant@golang.org>
Reviewed-by: Martin Möhrmann <moehrmann@google.com>