Change Arch repr to u32 instead of isize to support wasm32 targets

[maximebedard]

What type of PR is this?

/kind bug

What this PR does / why we need it:

This PR removes the clippy unportable warning and changes the internal representation of Arch from a isize to a u32. This makes the type portable across other architecture (e.g. WASM32). See the following example:

cargo build --target wasm32-unknown-unknown
   Compiling oci-spec v0.7.1
error: literal out of range for `isize`
    --> src/runtime/linux.rs:1112:22
     |
1112 |     ScmpArchX86_64 = 0xc000003e,
     |                      ^^^^^^^^^^
     |
     = note: the literal `0xc000003e` (decimal `3221225534`) does not fit into the type `isize` and will become `-1073741762isize`
     = note: `#[deny(overflowing_literals)]` on by default

Which issue(s) this PR fixes:

N/A

Special notes for your reviewer:

N/A

Does this PR introduce a user-facing change?

Anyone relying on Arch type to be a usize will have to explicitly convert it to a u32.