fail and fail_bounds_checks lang items should take `&'static str` not `*u8`

These lang items are currently defined as

``` rust
pub fn fail_(expr: *u8, file: *u8, line: uint) -> !
pub fn fail_bounds_check(file: *u8, line: uint, index: uint, len: uint) -> !
```

We convert the expr and file arguments to `&'static str` via `unsafe` internally, but we could just avoid this and have the compiler emit them as slices rather than C strings.

cc #12357.
