lint: _sbrk_r: change parameter type from size_t to ptrdiff_t

`incr` is of type `size_t` which is essentially a `unsigned int` type (according to ISO 1999 C it's an unsigned integer of at least 16 Bit). Since unsigned integers can't be smaller than zero the check is unnecessary (and will hopefully gets optimized away by the compiler anyways).
~~This PR removes the unnecessary check to silence `cppcheck`.~~
This PR changes the type of `incr` to `ptrdiff_t` (as suggested by @Kijewski ).
Added: changed this for `_sbrk_r` implementations for other platforms too.