Revert "picolibc: Update to picolibc-1.8.7"

This reverts commit 7011779 in order to
revert the SDK 0.16 LTS release to include Picolibc 1.8.6, which is the
Picolibc version used by the Zephyr LTS3.

Signed-off-by: Stephanos Ioannidis <[email protected]>

Author: stephanosio