coretests: move float tests from num to floats module and use a more flexible macro to generate them
This makes some progress on https://github.com/rust-lang/rust/issues/141726 by moving the float tests in `num` to `floats` and using a newer, more flexible macro to generate them. We also newly run these tests on f16 and f128 in const, and at runtime in Miri and for hosts where that works well enough.
I didn't yet deduplicate any tests or port the existing `floats::f*` tests to the macro, that can happen in a future PR.
try-job: x86_64-gnu-aux
0d2bf86284