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

It uses sigma0 protocol for getting pages. This library registers a
constructor that will fill the pool initially by getting pages from sigma0.
After setting up the pool no more pages will be requested at sigma0. If the
pool is empty mmap will fail.
The size of the pool can be influenced by setting

  const size_t libc_backend_sigma0_pool_mem_size = <value_in_bytes>;

in the program. The used unit is Bytes.
