[OpenCL] Disable C99 standard lib functions

The library functions defined in the C99 standard headers
are not available (OpenCL v1.2 s6.9.f). 

This change stops treating OpenCL builtin functions as standard C lib
functions to eliminate warning messages about printf format string.

Patch by Liu Yaxun (Sam)!

Differential Revision: http://reviews.llvm.org/D16812

llvm-svn: 260671