Use -fPIC on all arches; lua builds the object files into a shared lib.

Since the object files get built into a shared lib, using -fPIC on all
arches is the right thing to do, instead of adding it to each new arch that
comes along which doesn't accidentally allow non-PIC modules in shared libs.

PR: 207324
Approved by: mat(mentor)
Differential Revision: https://reviews.freebsd.org/D5362