Add vecadd-loop

This is the same kernel as vecadd but repeated in a for-loop many times so that
the runtime overhead at the startup is amortized.

tests/opencl/vecadd-loop/kernel.cl

@@ -0,0 +1,13 @@
+__kernel void vecadd_loop (__global volatile const float *A,
+	                    __global volatile const float *B,
+	                    __global volatile float *C)
+{
+  int gid = get_global_id(0);
+  float sum = 0.;
+  for (int i = 0; i < 500; i++) {
+      // int addr = gid + (i % 2);
+	  int addr = gid;
+      C[addr] += A[addr] + B[addr];
+  }
+  // C[gid] = sum;
+}