powerpc: Transition to Secure-PLT, like most other OSs (Toolchain part)

Summary:
Toolchain follow-up to r349350. LLVM patches will be submitted upstream for
9.0 as well.

The bsd.cpu.mk change is required because GNU ld assumes BSS-PLT if it
cannot determine for certain that it needs Secure-PLT, and some binaries do
not compile in such a way to make it know to use Secure-PLT.

Reviewed By: nwhitehorn, bdragon, pfg
Differential Revision: https://reviews.freebsd.org/D20598