It seems the webassembly backend does not want the exception that
`structFieldAlignmentExtern` makes for 128-bit integers. Perhaps that
logic should be modified to check if the target is wasm.

Without this, this branch fails the C ABI tests for wasm, causing this:
```
wasm-ld: warning: function signature mismatch: zig_struct_u128
>>> defined as (i64, i64) -> void in cfuncs.o
>>> defined as (i32) -> void in test-c-abi-wasm32-wasi-musl-ReleaseFast.wasm.o
```