Kernel+LibC: Build with basic -fstack-protector support

Use simple stack cookies to try to provoke an assertion failure on
stack overflow.

This is far from perfect, since we use a constant cookie instead of
generating a random one on startup, but it can still help us catch
bugs, which is the primary concern right now. :^)

Kernel/StdLib.cpp

@@ -183,4 +183,12 @@ void free(void* p)
 {
     return kfree(p);
 }
+
+extern u32 __stack_chk_guard;
+u32 __stack_chk_guard = (u32)0xc0000c13;
+
+[[noreturn]] void __stack_chk_fail()
+{
+    ASSERT_NOT_REACHED();
+}
 }