Another minor macro renaming for CHERI context-switch assembly: as we'll now
save 64-bit integers to the CHERI frame, not just capability registers, be
more explicit about register type in macro name to avoid confusion.
Remove sigframe in preference to an expanded cheri_frame that now holds a
copy of the capability cause register.  Save this value during involuntary
context switches from userspace, but don't restore it (currently).