Instead of using the data capability for the system-class object capability
to hold a global $c0, use it to hold an actual data structure pointer: the
sandbox to which the object capability was delegated.  $c0 was already being
installed from $pcc, meaning that $idc was actually unused.

Add a new in-libcheri API, cheri_enter_getsandbox(), which can be used to
return a sandbox capability pointer for use in the system class.

(This requires the most recent CHERI Clang/LLVM.)