[libc] Add malloc_usable_size, write general-purpose realloc

[ghaerr]

Adds size_t malloc_usable_size(void *ptr) which returns the allocated size of a malloc'd pointer.

Used that routine in writing a general-purpose realloc which then calls malloc/free with the correct size for memcpy. This general purpose realloc will allow for replacing malloc and free wholesale with __amalloc and __afree wholesale in the 8086 toolchain. The toolchain will be ready for updating after writing that wrapper function which also switches out large requests to fmemalloc. Next PR coming shortly.