Add Newlib dynamic reentrancy

[Dazza0]

Description

Currently, when configUSE_NEWLIB_REENTRANT is enabled, FreeRTOS will update Newlibs _impure_ptr on every context switch to point to the current task's reent struct. However, this behavior is no longer valid for SMP due to the fact that two or more cores can switch contexts, thus leading to a corrupted _impure_ptr (i.e., multiple cores cannot share the same _impure_ptr.

Newlib provides a __DYNAMIC_REENT__ feature where all newlib functions will call a __getreent() function to get the required reentrancy structure instead of referring to the _impure_ptr. In case of FreeRTOS, it should just return the current core's current task's reent struct.

This commit adds the following changes:

• A configNEWLIB_REENTRANT_IS_DYNAMIC option to enable Newlib dynamic reentrancy support in FreeRTOS SMP
• _impure_ptr is no longer modified by FreeRTOS when configNEWLIB_REENTRANT_IS_DYNAMIC is enabled
• Port should provide their own __getreent() (probably somewhere in freertos_tasks_c_additions.h. The function could look something like the following:

/**
 * @brief Get reentrancy structure of the current task
 *
 * - This funciton is required by newlib (when __DYNAMIC_REENT__ is enabled)
 * - It will return a pointer to the current task's reent struct
 * - If FreeRTOS is not running, it will return the global reent struct
 *
 * @return Pointer to a the (current taks's)/(global) reent struct
 */
struct _reent* __getreent(void) {
    // No lock needed because if this changes, we won't be running anymore.
    TCB_t *pxCurTask = xTaskGetCurrentTaskHandle();
    if (pxCurTask == NULL) {
        // No task running. Return global struct.
        return _GLOBAL_REENT;
    } else {
        // We have a task; return its reentrant struct.
        return &pxCurTask->xNewLib_reent;
    }
}