Finish implementing alternative libcheri sandbox setup and invoke paths based
on the CHERI-aware version of LLVM with __capability and CHERI builtins:

- Mark _chsbrt_invoke() as __attribute__((cheri_ccall)) so that callers will
  place code/data capabilities in $c1 and $c2 using the new ccall calling
  convention rather than the regular calling convention.  No change in the
  _chsbrt_invoke() implementation required.

- Implement sandbox_cinvoke() using _chsbrt_invoke().

- Implement sandbox_invoke() in terms of sandbox_cinvoke() when compiler
  support is available.  Use bzero() to generate zero-rights capabilities,
  although we might prefer another way of spelling this in the future.

- Fix bugs and loose ends in the __capability version.

libcheri can now use CHERI annotations/builtins instead of inline assembly.