Put a copy of the sandbox linker script in the libc_cheri tree, which
seems the most suitable centralised point to put it.  We will install
this to /usr/libdata where other linker scripts are installed.
Install the sandbox.ld linker script in /usr/libdata/ldscripts.

Update cheri_helloworld and cheritest helper so that they use the central
linker script and no longer need their own copies.
Further minor cleanups to the cheritest-helper makefile: no-longer required
paths, etc.