Kernel: Start implementing purgeable memory support

It's now possible to get purgeable memory by using mmap(MAP_PURGEABLE).
Purgeable memory has a "volatile" flag that can be set using madvise():

- madvise(..., MADV_SET_VOLATILE)
- madvise(..., MADV_SET_NONVOLATILE)

When in the "volatile" state, the kernel may take away the underlying
physical memory pages at any time, without notifying the owner.
This gives you a guilt discount when caching very large things. :^)

Setting a purgeable region to non-volatile will return whether or not
the memory has been taken away by the kernel while being volatile.
Basically, if madvise(..., MADV_SET_NONVOLATILE) returns 1, that means
the memory was purged while volatile, and whatever was in that piece
of memory needs to be reconstructed before use.

Libraries/LibC/mman.cpp

@@ -56,4 +56,11 @@ int shm_unlink(const char* name)
     int rc = syscall(SC_shm_unlink, name);
     __RETURN_WITH_ERRNO(rc, rc, -1);
 }
+
+int madvise(void* address, size_t size, int advice)
+{
+    int rc = syscall(SC_madvise, address, size, advice);
+    __RETURN_WITH_ERRNO(rc, rc, -1);
+}
+
 }