Prefer __has_feature(capabilities) to defined(__capability).
Implement a function cheri_systemcap_get() in libcheri, which allows an
application to request a cheri_object describing the callback environment
for the sandbox host.  These capabilities can then be delegated to sandboxes
to allow them to invoke system services.

While here, try to use __has_feature(capabilities) more thoroughly.
Teach cheritest to query and delegate system code and data capabilities to
the cheritest-helper sandbox; teach cheritest-helper to expect this.  No
cheritest-helper methods currently use the system capabilities, however.

While here, renumber up the capability registers used for certain tests from
3 to 11 to avoid reusing argument capabilities that will normally be used
for something else in the non-CHERI Clang/LLVM case.