The __DEVOLATILE pattern can't work with an all capablity ABI.
Enable the all capabilities ABI when using CHERI_STACK.
Build more helpers with CHERI_STACK.
Add -msoft-float to CHERI_CC to make sure we use it.
The sandbox abi superceeds cheri-stack-cap.
Make 64-bit atomic functions use clld/cscd.  Declare 32-bit functions,
but don't implement because they can't work reliably without cll/csc.
Add memset_c the old way.  In sandbox mode it should be an alias to memset.