Bump the default target CPU for i386-freebsd to i686

Summary:
Similar to what we have done downstream, some time ago:
https://svnweb.freebsd.org/changeset/base/353936

This followed some discussions on the freebsd-arch mailing lists, and
most people agreed that it was a better default, and also it worked
around several issues where clang generated libcalls to 64 bit atomic
primitives, instead of using cmpxchg8b.

Reviewers: emaste, brooks, rsmith

Reviewed By: emaste

Subscribers: arichardson, krytarowski, jfb, cfe-commits

Tags: #clang

Differential Revision: https://reviews.llvm.org/D83645