examples: littlefs: Use full libc

Examples should work with the default full libc to demonstrate that
users can fully utilize features offered by the toolchain.

Previously, Arm Compiler's MicroLib was used in the littlefs example
due to an issue with system I/O functions in the full libc. For details,
see the commit

    7192dcb8 examples: Build using a small C library

The issue has since been resolved by retargeting system I/O.

Signed-off-by: Lingkai Dong <lingkai.dong@arm.com>