[Flang] Define c_int_fast16_t and c_int_fast32_t for PowerPC. (#88292)

On Linux, PowerPC defines `int_fast16_t` and `int_fast32_t` as `long`.
Need to update the corresponding type, `c_int_fast16_t` and
`c_int_fast32_t` in `iso_c_binding` module so they are interoparable.