libarchive: Port upstream issue 320 fix

Port upstream commit 533e8fda (Rework the sign-extension to avoid
left-shift of an explicit negative number, 2013-06-29) into CMake.

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

5ee1297 libarchive: Port upstream issue 320 fix