riscv64: TODO: airBitCast [3]u8 to u24

[RetroDev256]

Zig Version

0.15.0-dev.764+2e3154428

Steps to Reproduce and Observed Behavior

Compile the following code with
zig build-obj repo.zig -fno-llvm -target riscv64-linux -mcpu baseline+v+zbb

var a: [3]u8 = undefined;

export fn b() u32 {
    return @as(u24, @bitCast(a));
}

You will observe the compile error

repo.zig:3:8: error: TODO: airBitCast [3]u8 to u24
export fn b() u32 {
~~~~~~~^~~~~~~~~~

Expected Behavior

No error

[Rexicon226]

Keep in mind that the self-hosted RISC-V backend is super immature, and I'm in the process of fully rewriting it. With the new legalization pass, it should become quite a bit more usable.

Opening issues about TODOs is a bit pointless at the moment, just because of how many there are to still be done. That being said though, I probably should make a tracking issue for all of the ones blocking it from becoming the default backend, similar to how the x86 backend did it.