picolibc: Proposed updates for picolibc

This aligns the picolibc for 0.18 with the version proposed for SDK
version 0.17.3 as well as for the Zephyr picolibc module.

Most of the Zephyr-relevant changes concern using picolibc as a
secondary C library and as a module, so there aren't any significant
changes for SDK version 0.18.

Signed-off-by: Keith Packard <[email protected]>

Author: keith-packard