On PowerPC, Secure-PLT by default for FreeBSD 13 and higher

Summary:
In https://svnweb.freebsd.org/changeset/base/349351, FreeBSD 13 and
higher transitioned to Secure-PLT for PowerPC.  This part contains the
changes in clang's PPC architecture defaults.

Reviewers: emaste, jhibbits, hfinkel

Reviewed By: jhibbits

Subscribers: wuzish, nemanjai, krytarowski, kbarton, MaskRay, jsji, shchenz, steven.zhang, cfe-commits

Tags: #clang

Differential Revision: https://reviews.llvm.org/D67119

llvm-svn: 372261