[CUDA] Tweak math forward declares so we're compatible with libstdc++4.9.

Summary:
See comments in patch; we were assuming that some stdlib math functions
would be defined in namespace std, when in fact the spec says they
should be defined in the global namespace.  libstdc++4.9 became more
conforming and broke us.

This new implementation seems to cover the known knowns.

Reviewers: rsmith

Subscribers: cfe-commits, tra

Differential Revision: http://reviews.llvm.org/D18882

llvm-svn: 265751