libc: minimal: Add strtoul implementation

This commit adds the strtoul function implementation that is licensed
BSD-3-Clause, which is an OSI-approved license, with the modifications
necessary for adoption into the Zephyr minimal C library.

Origin: Newlib
License: BSD 3-Clause
URL: git://sourceware.org/git/newlib-cygwin.git


Commit: 9042d0ce65533a26fc3264206db5828d5692332c
Purpose: strtoul function support in the minimal C library

Signed-off-by: Stephanos Ioannidis <root@stephanos.io>