Build cheritest.dump file, as we do for cheritest-helper.dump, since we seem
to need it pretty often.
Implement cheritest-helper routines for cheri_fd testing.  Also fix the
divide-by-zero test case.
Implement cheritest test cases for the four cheri_fd methods, which are
invoked by cheritest-helper on cheritest's behalf.
When printing '*'s next to CHERI stack frames to indicate liveness, include
the frame currently pointed to by the stack pointer.
For the time being, use NO_WERROR for libcheri and libc_cheri: we're
continuing to use $c0 so that we compile with older CHERI Clang/LLVM versions,
and therefore accept warnings with newer compiler warnings without failing the
build.