Multiple attributes are allowed on a single function

[jonas-schievink]

...leading to incorrect error messages:

#[exception]
#[entry]
fn SVCall() -> ! {
    loop {}
}

error: `#[entry]` function must have signature `[unsafe] fn() -> !`
  --> examples/bug.rs:17:1
   |
17 | #[exception]
   | ^^^^^^^^^^^^

The function already has the signature that the error message demands, and the span points at the other attribute instead of the function.