strnlen: Add our own function implementation

Implementing strnlen() function which is being
used by OTA for AWS library (ota_mqtt.c). This function
is not a standard C function which means it should be
user implemented. In case of GNU toolchain this function
is implemented in libc library. However, in case of
ARMClang this function is not implemented, that's
why it is defined in armclang_retarget.c file.

This function was introduced in ota_mqtt.c file
in the following commit for
aws-iot-device-sdk-embedded-c :
52f2d400eb5439017668817399cf3b342bf9210d

Signed-off-by: Ahmed.Ismail <Ahmed.Ismail@arm.com>