ggml-cpu: BMI2 is only available on amd64

[candrews]

MSVC doesn't define BMI2 so it must be defined by by CMake. It was being set whenever GGML_BMI2 is set, but that results in a failure (error LNK2019: unresolved external symbol __pdep_u64) when targeting x86 (aka 32 bit) Windows. The fix is to only set BMI2 when targeting amd64.