sys/cpu: modify x86 port to match what internal/cpu does

The way we call isSet is different in the two. Normalize x/sys
to match internal/cpu.

This was done using (mostly) gopls' quick fix, to ensure we
don't miss any of the transpositions.

Change-Id: I0b66152e194d46ed9299550e12641cc798f8b11b
Reviewed-on: https://go-review.googlesource.com/c/sys/+/731320
Reviewed-by: Cherry Mui <cherryyz@google.com>
Reviewed-by: Keith Randall <khr@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>