cmake: Do not include `zephyr_stdint.h` for native POSIX archs

This commit updates the Zephyr build system such that it does not
include the `zephyr_stdint.h`, which tries to define Zephyr's own type
system, when compiling for the native POSIX architecture.

The native POSIX architecture compiles with the host toolchain and
headers, and there can be conflicts if we arbitrarily define our own
type system (e.g. mismatch between the types and the specifiers defined
in `inttypes.h`).

Note that this is not meant to be a permanent fix; instead, it is meant
to serve as a temporary workaround until we no longer need to define
our own type system.

For more details, refer to the issue #37718.

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