libc: minimal: Add strstr implementation

This commit adds the strstr 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.

Note that this implementation is based on the size optimised version of
the newlib strcasestr function.

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


Commit: 9087163804df8af6dc2ec1f675a2341c25f7795f
Purpose: strstr function support in the minimal C library

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