generate valid C code for maths functions

This PR partially addresses this issue.

• When using CTarget, loopy will instead generate fmin() and fmax() according to C99 standard, rather than min and max.
• Relevant code in OpenCLTarget has been moved down to CTarget.
• Test added to test_target.py

Future work

• Change the behavior of pymbolic.primitives.Min and pymbolic.primitives.Max to generate corresponding function calls.