Properly support passing a packed struct to byteSwapAllFields

[SquareMan]

Closes #25054

This handles packed structs as a base case before recursing into struct fields.

[SquareMan]

I think I might need a little bit of help with the test failure. It looks like this fix has exposed another edge case with byteSwapAllFields: It can't deal with byte-swapping a struct that contains an inner struct with custom alignment. For example, trying to byteswap this struct will fail on master now:

    const A = extern struct {
        f0: u32,
        f1: extern struct {
            f0: u64,
        } align(4),
        f2: u32,
    };

std.zig.Server.Message.TestResults gets away with it right now because std.zig.Server.Message.TestResults.Flags is actually a packed struct, so it hits the packed struct special case when iterating fields and doesn't have to take a pointer to flags and recurse.

I'm not sure if there's a clean way to solve this. Making ptr anytype allows the alignment of the pointer to be captured and fix the error, but that's pretty hacky and I assume not acceptable.

[SquareMan]

I've come up with one solution here of renaming byteSwapAllFields to byteSwapAllFieldsAligned and modified the signature to accept an alignment parameter. Then I converted byteSwapAllFields into a wrapper function that will call byteSwapAllFieldsAligned with standard aligment for 99% use case where the root struct you're trying to byteswap is itself aligned in memory.

Open to suggestions on how to improve this further, if necessary.