[release-branch.go1.9] cmd/cgo: avoid using common names for sniffing

Current code uses names like "x" and "s" which can conflict with user's
code easily. Use cryptographic names.

Fixes #21668

Change-Id: Ib6d3d6327aa5b92d95c71503d42e3a79d96c8e16
Reviewed-on: https://go-review.googlesource.com/59710


Reviewed-by: Ian Lance Taylor <iant@golang.org>
Reviewed-on: https://go-review.googlesource.com/59730


Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Hiroshi Ioka <hirochachacha@gmail.com>
Reviewed-by: Chris Broadfoot <cbro@golang.org>
Reviewed-on: https://go-review.googlesource.com/70849
Run-TryBot: Russ Cox <rsc@golang.org>