switch to disable intrinsic functions about fpm

[MikeLankamp]

Hi @Klummel69. I can see the value in supporting those platforms. I'd indeed prefer one of the "bit hack" solutions over the naive loop, but I'd rather avoid adding preprocessor macros, so
a) I'd want to benchmark the difference (it's possible the bit hacks are faster), and
b) I'd like the code to be generalized to the templated type.
I'll have to think about it, actually understand the bit hacks and experiment a bit.