powerpc: Transition to Secure-PLT, like most other OSs

Summary:
PowerPC has two PLT models: BSS-PLT and Secure-PLT. BSS-PLT uses runtime
code generation to generate the PLT stubs. Secure-PLT was introduced with
GCC 4.1 and Binutils 2.17 (base has GCC 4.2.1 and Binutils 2.17), and is a
more secure PLT format, using a read-only linkage table, with the dynamic
linker populating a non-executable index table.

This is the libc, rtld, and kernel support only. The toolchain and build
parts will be updated separately.

Reviewed By: nwhitehorn, bdragon, pfg
Differential Revision: https://reviews.freebsd.org/D20598
MFC after: 1 month