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.

Kernel/Syscall.h

@@ -144,7 +144,9 @@ typedef u32 socklen_t;
     __ENUMERATE_SYSCALL(module_unload)          \
     __ENUMERATE_SYSCALL(detach_thread)          \
     __ENUMERATE_SYSCALL(set_thread_name)        \
-    __ENUMERATE_SYSCALL(get_thread_name)
+    __ENUMERATE_SYSCALL(get_thread_name)        \
+    __ENUMERATE_SYSCALL(madvise)                \
+    __ENUMERATE_SYSCALL(purge)
 
 namespace Syscall {