INIT:ARM + rtos: Align pre-main initialization steps between TCs

Various toolchains supported in MBED don't followthe same initialization
steps. This can have impacts on platform behavior.

For STM32, it is needed to call the HAL_Init() _after_ the  RAM has been
initialized (sdata from flash / zero initialized data) and _before_ the C++
objects are being created, especially if those objects require support
of tickers for instance.

In GCC and IAR, this was done in previous commit to avoid HAL_Init()
to be called twice.

In ARM this there is no hook defined in MBED yet to place the call.
The proposal is to take benefit of the library's
_platform_post_stackheap_init function that is going to be called before
__rt_lib_init where the C++ object init is done (__cpp_initialize__aeabi_)

In case of mbed with rtos, the __rt_entry is redefined so we need to add
the call to _platform_post_stackheap_init.

This series should solve issue reported here:
STM32 (At least F401) breaks if Tickers are activated in a global object #2115

Author: Laurent MEUNIER

rtos/rtx/TARGET_CORTEX_M/RTX_CM_lib.h

@@ -746,6 +746,7 @@ void pre_main (void)
 __asm void __rt_entry (void) {
 
   IMPORT  __user_setup_stackheap
+  IMPORT  _platform_post_stackheap_init
   IMPORT  os_thread_def_main
   IMPORT  osKernelInitialize
 #ifdef __MBED_CMSIS_RTOS_CM
@@ -766,6 +767,7 @@ __asm void __rt_entry (void) {
   /* Ignore return value of __user_setup_stackheap since
    * this will be setup by set_stack_heap
    */
+  BL      _platform_post_stackheap_init
   BL      osKernelInitialize
 #ifdef __MBED_CMSIS_RTOS_CM
   BL      set_stack_heap