GetCacheSizesMacOSX(): use consistent types. (#667)

I have absolutely no way to test this, but this looks obviously-good.

This was reported by Tim Northover @TNorthover in
http://lists.llvm.org/pipermail/llvm-commits/Week-of-Mon-20180903/584223.html

> I think this breaks some 32-bit configurations (well, mine at least).
> I was using Clang (from Xcode 10 beta) on macOS and got a bunch of
> errors referencing sysinfo.cc:292 and onwards:

> /Users/tim/llvm/llvm-project/llvm/utils/benchmark/src/sysinfo.cc:292:47:
> error: non-constant-expression cannot be narrowed from type
> 'std::__1::array<unsigned long long, 4>::value_type' (aka 'unsigned
> long long') to 'size_t' (aka 'unsigned long') in initializer list
> [-Wc++11-narrowing]
>   } Cases[] = {{"hw.l1dcachesize", "Data", 1, CacheCounts[1]},
>                                               ^~~~~~~~~~~~~~
>
> The same happens when self-hosting ToT. Unfortunately I couldn't
> reproduce the issue on Debian (Clang 6.0.1) even with libc++; I'm not
> sure what the difference is.