[PowerPC64] Fix broken kernel modules due to LLD 9+ TOC optimization

LLD9 introduced a TOC optimization that isn't compatible with kernel dynamic
linker causing panic when loading kernel modules (pf, linuxkpi etc.)

This patch disables TOC optimization when building kernel modules.

Submitted by: Alfredo Dal'Ava Junior <alfredo.junior@eldorado.org.br>
Approved by: jhibbits (mentor)
Differential Revision: https://reviews.freebsd.org/D22317