libc: picolibc: disable prompt for PICOLIBC_USE_MODULE...

...when toolchain does not have native picolibc support.
Without native support, picolibc needs to be built from
the module. Disabling the prompt means this kconfig
takes on whatever default value specified, which is
to build source from module when there is no native
support.

Signed-off-by: Daniel Leung <daniel.leung@intel.com>