If USE_CHERI is supplemented with USE_CHERI_STACK in a Makefile, then pass
suitable LLVM compiler arguments to enable use of $c11 as a stack capability.
Add commented out USE_CHERI_STACK options to cheritest-helper and libc_cheri;
unfortunately, due to compiler bugs, these will not yet compile.
Set up (and later clear) a $c0-based stack capability in $c11 when entering
a cheritest sandbox.