lib: picolib: put stdio variables into libc partition
lib: picolib: put stdio variables into libc partition

This adds the necessary modifier to the stdin/stdout/stderr
variables in picolib, and putting into the z_libc_partition.
This allows userspace applications to utilize these variables
for console I/O.

Fixes #51343

Signed-off-by: Daniel Leung <d94e04d205b5962d7873a139ef298b03c1717f72@intel.com>

zephyrproject-rtos/zephyr,finikorg/zephyr,galak/zephyr,finikorg/zephyr,finikorg/zephyr,galak/zephyr,finikorg/zephyr,galak/zephyr,zephyrproject-rtos/zephyr,zephyrproject-rtos/zephyr,finikorg/zephyr,galak/zephyr,galak/zephyr,zephyrproject-rtos/zephyr,zephyrproject-rtos/zephyr