[mono][interp] Fix a few local store warnings (#66766)

[mono][interp] Fix a few local store warnings (#66766)

* [mono][interp] Fix warning when storing function pointers

* [mono][interp] Fix storing of R4 or R8 into local of different size

The IL execution stack operates on generic floating type F, which can be stored in local of both float32 or float64. We however operate on the real types and need to add implicit conversions if necessary.

* [mono][interp] Fix ckfinite

It was previously handling only R8 correctly.

* [mono][interp] Add implicit conversion between nint and int32

* [mono][interp] Change warning to error