It is very common, and well-defined, for a pointer on one side of a C ABI
to have a different but compatible element type. Examples include:

- `char*` vs `uint8_t*` on a system with 8-bit bytes
- `const char*` vs `char*`
- `char*` vs `unsigned char*`

Without this flag, Clang would invoke UBSAN when such an extern
function was called.

Might be nice to file an upstream issue and find out if there is a more
precise way to disable the problematic check.
`-fsanitize-cfi-icall-generalize-pointers` looks promising according to
the documentation, but empirically it does not work.