build: make Zephyr stdint convention enforcing configurable

There are  few cases where removing this enforcement is desirable:

- linking against binary C++ libs with incompatible type mangling

- linking against system provided headers i.e. native_posix

- compiling with legacy code that assumes a different convention

So let's create a Kconfig symbol for it. This is IMHO a good compromize
compared to using the %"PRId32" abomination everywhere otherwise.

Signed-off-by: Nicolas Pitre <npitre@baylibre.com>