allow @asyncCall with a real @Frame(func) rather than byte slice

[andrewrk]

const std = @import("std");
const expect = std.testing.expect;

test "@asyncCall with actual frame instead of byte buffer" {
    var frame: @Frame(func) = undefined;
    var result: i32 = undefined;
    const ptr = @asyncCall(&frame, &result, func);
    resume ptr;
    expect(result == 1234);
}
fn func() i32 {
    suspend;
    return 1234;
}

expected to pass. actual output:

/home/andy/dev/zig/build/test.zig:7:29: error: expected type '[]u8', found '*@Frame(func)'
    const ptr = @asyncCall(&frame, &result, func);
                            ^

The point of this is to pass a result location to the function so that the result never has to get copied.