[WIP][CHERI-RISCV-V] Add option to zero ra/cra slot on return

[arichardson]

If -mllvm -zero-return-slot-after-restore is passed to clang, we will now
write zero to the (c)ra stack slot. This flag is experimental as it
currently makes some assumptions that may not necessarily always be true.

[arichardson]

This does not seem to work yet:

start_init: trying /sbin/init
pid 19 (sh), uid (0): Failed to open coredump file 'sh.core', error=30
pid 19 (sh), jid 0, uid 0: exited on signal 34
2020-06-25T12:46:32.321835+00:00  init 1 - - /bin/sh on /etc/rc terminated abnormally, going to single user mode
Enter full pathname of shell or RETURN for /bin/sh: