[CUDA] Implemented device-side support functions in <cmath>.

CUDA expects math functions in std:: namespace to work on device side.
In order to make it work with clang without allowing device-side code
generation for functions w/o appropriate target attributes, this patch
provides device-side implementations for <cmath> functions. Most of
them call global-scope math functions provided by CUDA headers. In few
cases we use clang builtins.

Tested out-of tree by compiling and running thrust's unit_tests.
https://github.com/thrust/thrust/tree/master/testing

Differential Revision: http://reviews.llvm.org/D16593

llvm-svn: 258880