libc: minimal: Add PRIxMAX macros for [u]intmax_t

This commit adds the missing `PRIxMAX` macros for the C99 `intmax_t`
and `uintmax_t` types:

  PRIdMAX, PRIiMAX, PRIoMAX, PRIuMAX, PRIxMAX, PRIXMAX

Note that the `PRIxMAX` macros specify the `ll` size modifier because
the type of the `intmax_t` for the minimal libc is defined as that of
the `int64_t`, which is always overridden to `long long int` by
`zephyr_stdint.h`; for more details, refer to the GitHub PR #29876,
which deliberately introduced this scheme.

In the future, this scheme will need to be reworked such that the
minimal libc `stdint.h` defines `intmax_t` as `__INTMAX_TYPE__`, and
the `inttypes.h` resolves the corresponding format specifier.

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