mem pool size: Fix calculation of memory pool size for portability ...

Replace the hardcoded value calculation of the memory pool block size
with the RTX preprocessor macro (via a shim layer).
This is in preparation for the replacement of the direct access of RTX
functionalities in Mbed OS with an access via CMSIS.

rtos: use RTX identifier ...

os_thread_t and family are internal and should not be used (thus we included
internal header file). Instead, use those that are exposed via rtx_os.h file.

Remove use of internal RTX types ...

Make calls to cmsis-os to get thread state, stack size, and max stack
usage rather than accessing internal RTX data directly. Wrap RTX5
specific code in OS_BACKEND_RTX5.

Also refactor the code to use mbed types rather than RTX types:
os_timer_t -> mbed_rtos_storage_timer_t
os_event_flags_t -> mbed_rtos_storage_event_flags_t
osRtxMutex_t -> mbed_rtos_storage_thread_t

Move Cortex specific RTX behind TARGET_CORTEX ...

Update directory structure to include RTX for only cortex targets, and
for all cortex targets. This patch accomplishes this by moving mbed-os
specific RTX files and RTX itself into rtos/TARGET_CORTEX along with
removing TARGET_CORTEX_M from the RTX5 directory.

The old directory structure:

rtos/rtx5/<mbed-os specific RTX files>
rtos/rtx5/TARGET_CORTEX_M/*
rtos/rtx4/*
rtos/<mbed-os specific RTX files>

Is re-arranged to:

rtos/TARGET_CORTEX/rtx5/*
rtos/TARGET_CORTEX/rtx4/*
rtos/TARGET_CORTEX/<mbed-os specific rtx files>

This both encapsulates RTX code more cleanly and makes it easier to
experiment with non-cortex cmsis-os2 backends, such as a posix
based cmsis-os2 backend.

Note - A potentially better name for the CORTEX_M directory would be
something like FEATURE_RTX5 since this directory only contains RTX5
related files. This cannot be done because there is not an easy way
to turn this feature on, since it cannot be done from mbed_lib.json.