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

tool/emacs/cosmo-c-builtins.el

@@ -45,7 +45,8 @@
          '("__builtin_va_list"))
 
         (gcc-builtin-functions
-         '("__builtin_va_start"
+         '("__builtin_strlen"
+           "__builtin_va_start"
            "__builtin_va_arg"
            "__builtin_va_end"
            "__builtin_add_overflow"