WIP: Use fmap2 when implementing mmap on Redox

Uses fmap2, which allows also accepting an address as a hint to where the userspace program wants the memory reservation to take place. It also allows pinning the mapping to a specific address, by using the MAP_FIXED flag also introduced in this MR. This will allow ld.so to properly map the Global Offset Table to the right address.

Depends on syscall!52 (merged).