sys: cbprintf: Fix performance-no-int-to-ptr warning

clang-tidy reports a performance-no-int-to-ptr warning due to the cast
(const char *)(uintptr_t)(v).

Previously, only char * was cast to const char *, but there's no downside
to constifying all pointer types.

This change updates the Z_CONSTIFY macro to apply const consistently,
which even aligns better with its name and resolves the warning.

Signed-off-by: Tim Pambor <tim.pambor@codewrights.de>