Add support for newlib dynamic reentrancy (#496)

Add support for newlib dynamic reentrancy (#496)

Previously, newlib's _impure_ptr was updated on every context switch
to point to the current task's _reent structure.

However, this behavior is no longer valid on multi-core systems due
to the fact that multiple cores can switch contexts at the same time,
thus leading to the corruption of the _impure_ptr.

However, Newlib can be compiled with __DYNAMIC_REENT__ enabled which
will cause newlib functions to call __getreent() instead in order to
obtain the required reent struct.

This commit adds dynamic reentrancy support to FreeRTOS:

- Added a configNEWLIB_REENTRANT_IS_DYNAMIC to enable dynamic reentrancy support
- _impure_ptr is no longer updated with reentrancy is dynamic
- Port must provide their own __getreent() that returns the current task's reent struct