Use more cryptic names for local variables inside C function wrappers.

Fixes #23356

Change-Id: Ia6a0218f27a13be14f589b1a0facc9683d22ff56
Reviewed-on: https://go-review.googlesource.com/86495
Run-TryBot: Tobias Klauser <tobias.klauser@gmail.com>
Reviewed-by: Tobias Klauser <tobias.klauser@gmail.com>
Reviewed-by: Ian Lance Taylor <iant@golang.org>