Add C++ demangling to privileged runtime

Cosmo will now print C++ symbols correctly in --ftrace logs and
backtraces. Doing this required reducing the memory requirement
of the __demangle() function by 3x. This was accomplished using
16-bit indices and 16-bit malloc granularity. That puts a limit
on the longest symbol we can successfully decode, which I think
would be around 6553 characters long, given a 65536-byte buffer

libc/runtime/quick_exit.c

@@ -21,6 +21,12 @@
 
 void __funcs_on_quick_exit(void);
 
+/**
+ * Exits process faster.
+ *
+ * @param exitcode is masked with 255
+ * @noreturn
+ */
 wontreturn void quick_exit(int code) {
   if (_weaken(__funcs_on_quick_exit))
     _weaken(__funcs_on_quick_exit)();