GCC most likely.

Is it possible to use LLVM? That generally compiles faster code for me than GCC, although that may be because my LLVM version is slightly newer.

Then it would be testing how GCC compiles C code and not Nimrod, right?

I can see what you are trying to say. But it would still be testing the speed of Nimrod, whether the Nimrod compiler compiles Nimrod code to C or to Assembly is irrelevant. The fact is that Nimrod is not C, and we are interested in the speed that Nimrod can achieve.