Fix a typo in de5b3861; using `$ver` to determine linux major/minor
versions would produce those for clang, use `$linuxver` instead.

Updates #12898.

Change-Id: I2c8e84ad02749fceaa958afd65e558bb0b08dddb
Reviewed-on: https://go-review.googlesource.com/17323Reviewed-by: Ian Lance Taylor <iant@golang.org>
Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>