Begin building the scaffolding to allow recursive invocation of unsandboxed
code from within a sandbox: implement assembly landing code for CCall in
__cheri_entry() that invokes C function cheri_entry() in libcheri.  This is
based on similar code in our sandboxed helpers, but without heap
initialisation specific to that environment.  Further work is required to
properly set up a stack, however, so not yet ready for use.