This includes Cortex A support and directory reshuffle.

mbed OS used older RTX4 version and with osThreadDef accepting only 3
parameters, to preserve compatibility we hardcode the 'instances'
parameter to 1.

This implementation of timer conflicts with the default ptim, we will
keep the default and let the timer override the implmenetation if
needed.

This fixes #4602 issue, mbed_main should be invoked right before the real main

Only set the SVCall priority if uVisor is not present. If uVisor is
present, keep using whatever priorities it has already set up.

Add the OsEventObserver mechanism. A client interested in receiving
notifications on certain OS events can register to receive notifications
with osRegisterForOsEvents. This is useful for clients like the secure
memory allocator, which observes thread switching events in order to
swap in and out different memory allocator objects.

OsEventObserver objects expect a context to be maintained per thread on
their behalf. Add this context to the thread control block and extend
the thread creation functions with the ability to supply a context.

uVisor doesn't set the PSP of the target thread. The RTOS sets the PSP
of the target thread from the target thread's TCB. However, when
interrupts of higher priority than PendSV happen between the call to
uVisor to switch boxes, and the RTOS setting PSP, the uVisor vIRQ
interrupt handler will attempt to use an invalid PSP (the PSP from
before the box and thread switch). This leads to a crash. Make box and
thread switching atomic by disabling interrupts immediately before the
box switching until immediately after the new PSP is set.

These tests are not compatible with Cortex A due to __get_MSP usage.

Remove redefinition of __CLZ and __SSAT.

Signed-off-by: Tony Wu <tung7970@gmail.com>