This backend provides very simple mmap functionality, as such backing
malloc & co.

Memory is supplied by the program itself, by specifying something like this:

char libc_backend_self_mem[64 << 10] __attribute__((aligned(L4_PAGESIZE)));
const int libc_backend_self_mem_size = sizeof(libc_backend_self_mem);

to reserve 64kb of memory for the allocator.
