toolchain: Override __[U]INT{FAST,LEAST}N__ macros

This commit overrides the following toolchain type macros to match the
respective base types overridden by the `zephyr_stdint.h`:

* __INT_FAST32_TYPE__
* __INT_FAST64_TYPE__
* __UINT_FAST32_TYPE__
* __UINT_FAST64_TYPE__

* __INT_LEAST32_TYPE__
* __INT_LEAST64_TYPE__
* __UINT_LEAST32_TYPE__
* __UINT_LEAST64_TYPE__

This is in preparation for adding the `PRIx{FAST,LEAST}N` macros in the
Zephyr minimal libc `inttypes.h`, which assumes the types defined by
the `zephyr_stdint.h`.

Signed-off-by: Stephanos Ioannidis <root@stephanos.io>