Miri: handling of SNaN inputs in f*::pow operations

[LorrensP-2158466]

fixes miri/#4286 and related to #138062 and miri/#4208.

For the following cases of the powf or powi operations, Miri returns either 1.0 or an arbitrary NaN:

• powf(SNaN, 0.0)
• powf(1.0, SNaN)
• powi(SNaN, 0)

Also added a macro in miri/tests/pass/float.rs which conveniently checks if both are indeed returned from such an operation.

Made these changes in the rust repo so I could test against stdlib, since these were impacted some time ago and were fixed in #138062. Tested with:

env MIRIFLAGS=-Zmiri-many-seeds ./x miri --no-fail-fast std core coretests -- f32 f64

This was successful. This does take a while, so I recommend using --no-doc and separate use of f32 or f64

The pr is somewhat split up into 3 main commits, which implement the cases described above. The first commit also introduces the macro, and the last commit is just a global refactor of some things.

r? @RalfJung