Implement a cheri_helloworld-helper back end that can perform "hello
world" in sandbox three different ways: the libcheri system-class
hello world service, the system-class puts service, and the
cheri_fd-class file-descriptor write service.  No longer depends on
the cheritest-helper back end and hence gives a somewhat simpler
example application.
Centralise our implementation of the CCall entry point in csu/cheri,
previously present in each sandboxed binary in the form of chsbrt.S.
I've updated cheri_helloworld and cheritest's helpers, but we need
to apply this change to other demos.  We also need to centralised
the sandbox-linker script.