Implement trunc builtin.

OpenCL C lang says that trunc rounds towards zero.
llvm.trunc.* intrinsic rounds to integer not larger in magnitude.
These definitions are equivalent.

Patch by: Jan Vesely

Reviewed-by: Tom Stellard <thomas.stellard@amd.com>
Signed-off-by: Jan Vesely <jan.vesely@rutgers.edu>