Use register names that gas is happy with for n64 (not warned about by
LLVM-MC); improve comments.
Begin introducing CHERI LLVM / __capability support in libcheri by providing
alternative code paths (ifdef'd USE_CHERI_C) performing the same functions
with compiler help.  Some parts become dramatically prettier -- e.g., sandbox
setup.  Others remain a work-inprogress -- e.g., the invoke path, where the
non-compiler supported case is straight forward, but a __capability-aware
version will require careful thinking in order to avoid treading on the
compiler's toes (especially with respect to $c1/$c2, the object capability to
be invoked).
Pad out stack frame used to save general-purpose registers around CCall in
order to ensure 32-byte alignment required to save capability registers.

Restore $c0 before using it to restore capability registers from the stack
after CReturn.