Added minval_f32/maxval_f32 functions for the OpenCL target

I found that OpenCL program, which was generated from the following code, does not compile:

```c++
Halide::Image<float> input(1024, 1024);

int winSize = 7;

Halide::Var x("x"), y("y");
Halide::Func minFilter("minFilter");
Halide::RDom minDomain(-winSize, 2 * winSize + 1, -winSize, 2 * winSize, "minLoop");

minFilter(x, y) = minimum(minDomain, input(x + minDomain.x, y + minDomain.y));

minFilter.gpu_tile(x, y, 16, 16);

Halide::Image<float> output(input.width() - 2 * winSize, input.height() - 2 * winSize);
output.set_min(winSize, winSize);

minFilter.realize(output);
```

it says that *maxval_f32* function definition is missed, so I've copied *minval_f32/maxval_f32* functions from the C code generator.