Disable the newlib-nano stubs code when picolibc is in use
Signed-off-by: Keith Packard <keithp@keithp.com>
---
v2:
Squash fixes in
v3:
call stdio_init in _PICOLIBC_ mode to initialize uart
v3:
Remove call to stdio_init from nanostubs_init, always
call from cpu_init.