Fix PPC64 kernel build with clang8 + lld8

This patch fixes the following lld link errors:

• unsupported dynamic relocations on read-only sections
• out-of-range TOC references

Submitted by: git_bdragon.rtk0.net
Reviewed by: jhibbits, luporl
MFC after: 2 weeks
Differential Revision: https://reviews.freebsd.org/D19352