libarchive: Port upstream issue 320 second fix

Port upstream commit 6cf33c93 (Issue 320: Rewrite (again) to avoid
the left shift that CLang dislikes so much, 2013-12-07) into CMake.

Inspired-by: Tim Kientzle <kientzle@freebsd.org>
Merge topic 'libarchive-left-shift-fix' into next

61b39af libarchive: Port upstream issue 320 second fix