syntax: Fix span on illegal ABI errors

Fixes #8537
Fixes #14499 (duplicate of #8537)

Old:
```rust
test.rs:2 	pub extern "xxxxx" fn add(x: int, y: int) -> int {
          	                   ^~
```

New:
```rust
test.rs:2 	pub extern "xxxxx" fn add(x: int, y: int) -> int {
          	           ^~~~~~~
```