picolibc: clang cmake fixes (for module), atomics fix, wchar_t bits

Move picolibc to current main branch version as that includes fixes
needed for the zephyr module when building with clang, wchar_t support
in stdio and a fix for the atomics used in ungetc/getc.

Signed-off-by: Keith Packard <keithp@keithp.com>

Author: keith-packard