[SystemZ] Implement vector rotate in terms of funnel shift

Clang currently implements a set of vector rotate builtins
(__builtin_s390_verll*) in terms of platform-specific LLVM
intrinsics.  To simplify the IR (and allow for common code
optimizations if applicable), this patch removes those LLVM
intrinsics and implements the builtins in terms of the
platform-independent funnel shift intrinsics instead.

Also, fix the prototype of the __builtin_s390_verll*
builtins for full compatibility with GCC.