`read_f64_into` includes non-portable assertion

[workingjubilee]

this assertion is not universally correct, unfortunately:

https://github.com/BurntSushi/byteorder/blob/1.5.0/src/lib.rs#L1271

u64 has a higher alignment than f64 on AIX.

The alignment for f64 is currently misreported by Rust as equal to u64 on AIX for deeply convoluted reasons that require multiple levels of compiler fixes.

cc @amy-kwan