Add a workaround for 128 bit switches

[FractalFir]

Due to a libgccjit limitation, 128 bit switches are currently not supported.

As a workaround, this PR:

1. Detects switches on 16 byte(128 bit) values.
2. Replaces such switches with an "if ladder".

Effectively, we turn this:

let val = black_box(64_u128);
match val {
    0 => return 1,
    1 => return 2,
    64 => (),
    _ => return 3,
}

Into this:

let val = black_box(64_u128);
if val == 0{
  return 1;
}
else if val == 1{
   return 2;
}
else if val ==   64 {
} else {
   return 3; 
}

This is less efficient than a switch, but it only applies to previously unsupported edge case: 128 bit switches. So, the small perf penalty is acceptable in this case.

Besides that, this PR contains a regression test for this issue, and a few tiny changes to minicore.

1. Added support for the black_box intrinsic - it is strictly necessary for correct tests. Even now, MIR optimizations sometimes optimize the crash I test for away. Using black_box guarantees we will not miss any issues in the future.

2. Added copy impls for i64, u128, and i128. The u128 impl was needed to properly test 128 bit switches. Adding one for i128 too seemed reasonable.

[antoyo]

Thanks a lot!
Nice work as always.

A few nitpicks.