Fix build on aarch64 and with Clang 6. The modified patch explains
the error message and what is changed. Bump PORTREVISION because
it contains a code change.

kde@ bumped into this while building x11/kde4 for aarch64.

Build on 12-CURRENT aarch64, 11-STABLE amd64 and 10.3 i386.

Approved by: tcberner (mentor)
Differential Revision: https://reviews.freebsd.org/D14008