Define a feature for __float128 support in the PPC back end

This patch corresponds to review:
http://reviews.llvm.org/D15117

In preparation for supporting IEEE Quad precision floating point,
this patch simply defines a feature to specify the target supports this.
For now, nothing is done with the target feature, we just don't want
warnings from the Clang FE when a user specifies -mfloat128.
Calling convention and other related work will add to this patch in
the near future.

llvm-svn: 255642