Various improvements to libcheri's support for the system class:

- Point the stack pointer at the top, rather than the bottom, of the
  memory-mapped invocation stack.

- Configure $c0 before, rather than after, loading the stack pointer from
  memory using MIPS instructions.

- Add a sample printf() in cheri_entry() to prove to ourselves that we can.
Define LIBC_CHERI for our new libc_cheri.
Link the cheritest-helper binary against libc_cheri, since we will need to
invoke sandboxes using its cheri_invoke() routine.
Implement a CHERITEST_HELPER_OP_SYSCAP operation in cheritest-helper,
which will invoke the system code/data capability using cheri_invoke().
Add an "invoke_syscap" command to cheritest.