Update float.h "# else // GPU" comments

[anthonydito]

This confused me for a couple minutes, so thought I would throw up a PR to update these comments.

#ifdef PBRT_IS_GPU_CODE
< GPU Code >
#else  // GPU
< CPU Code >
#endif

I think the comment next to the else is supposed to say "CPU" since that is the block that is happening here.

#ifdef PBRT_IS_GPU_CODE
< GPU Code >
#else  // CPU
< CPU Code >
#endif

This PR simply updates those comments.

[mmp]

Yep, that's better-thanks!