Enable per-library allocator statistics

Enable per-library allocator statistics

Instruments the library so that per-library allocator statistics are
possible. We make sure that code that uses `malloc()`, `free()`, and
company will resolve `uk_alloc_get_default()` within their compilation
scope so that the individual wrapper allocators can be used.
For this purpose, we declare the allocator functions as `static inline`
within the corresponding headers (like `<stdlib.h>`). Since the libc
headers are modified, we apply the new behavior only when
`CONFIG_LIBUKALLOC_IFSTATS` is switched on.

Signed-off-by: Simon Kuenzer <simon.kuenzer@neclab.eu>
Reviewed-by: Cezar Craciunoiu <cezar.craciunoiu@gmail.com>