Implement initial Altivec support

Summary:
This adds the register plumbing, as well as register reading in FreeBSD core
dumps.  Further work on the POSIX/FreeBSD ProcessMonitor is required in order to
support ptrace access to these registers.

Reviewers: tfiala, emaste

Reviewed By: emaste

Subscribers: emaste, lldb-commits

Differential Revision: http://reviews.llvm.org/D7039

llvm-svn: 228278