Malformed Wasm binaries with `@extern`'s `std.builtin.GlobalLinkage`

Version is 0.10.0-dev.4176+6d7b0690a (stage2).

pub fn main() void {
    const a = @extern(*const fn ([*]const u8) void, .{
        .name = "a",
        .library_name = "b",
        .linkage = .Strong, // or `.Weak`
        .is_thread_local = false,
    });
    a("hello");
}

zig build-exe -target wasm32-freestanding-none -O ReleaseSmall x.zig

This seems to generate malformed Wasm?

$ wasm2wat x.wasm
x.wasm:0000048: error: type mismatch at end of function, expected [] but got [i32]

I've updated this for stage2. Only one bug persists now.

[Luukdegram]

Thanks to #14102 this now generates the proper undefined symbol 'a' linker error as it does with other platforms such as ELF/MachO. When forced with --import-symbols it does however generate a proper Wasm binary now:

> wasm-validate x.wasm
> $?
0

Note this imports a function pointer, not a function symbol, and therefore will not generate an import entry in the WebAssembly module.