[PowerPC] The builtins load8r and store8r are Power 7 plus.

This patch makes sure that the builtins __builtin_ppc_load8r and
__ builtin_ppc_store8r are only available for Power 7 and up.
Currently the builtins seem to produce incorrect code if used for
Power 6 or before.

Reviewed By: nemanjai, #powerpc

Differential Revision: https://reviews.llvm.org/D110653