Remove now-obsolete comments about struct cheri_frame.
Make struct cheri_frame available to userspace, as we will expose this to
signal handlers and other ucontext users.  Note that this will make
cheri_frame part of the ABI and hence more sensitive to change.
Add pointer and length fields for coprocessor 2 state to the MIPS mcontext
and sigcontext structures -- unfortunately, reserved space is represented
in terms of 4-byte integers, making these additions consume different
numbers of reserved fields depending on whether we are on 32-bit or 64-bit
MIPS.  This is modelled on similar extensions to the x86 ucontext structure
where extended floating-point state is stored externally.  It's not yet
clear to what extent we can allow these extensions to work with non-CHERI
CP2s on other MIPS processors (e.g., crypto coprocessors on Cavium).