Kernel+LibC: Add MAP_RANDOMIZED flag for sys$mmap()

This can be used to request random VM placement instead of the highly
predictable regular mmap(nullptr, ...) VM allocation strategy.

It will soon be used to implement ASLR in the dynamic loader. :^)

Kernel/VM/RangeAllocator.h

@@ -87,6 +87,7 @@ public:
 
     Optional<Range> allocate_anywhere(size_t, size_t alignment = PAGE_SIZE);
     Optional<Range> allocate_specific(VirtualAddress, size_t);
+    Optional<Range> allocate_randomized(size_t, size_t alignment);
     void deallocate(const Range&);
 
     void dump() const;