[PPC] FE support for generating VSX [negated] absolute value instructions

Includes new built-in, conversion of built-in to target-independent intrinsic
and update in the header file. Tests are also updated. There is a second part in
the backend for which I will post a separate code-review. BACKEND PART SHOULD BE
COMMITTED FIRST.

Phabricator: http://reviews.llvm.org/D17816
llvm-svn: 263051