Take a very rough cut at providing CHERI adaptations to mips branch
emulation, which is used in a number of obscure cases -- such as unaligned
load emulation in a branch-delay slot.  In general, user code won't ever
need this, but sometimes it does.  A number of caveats -- as with other
instruction emulation (e.g., unaligned access emulation), borrowing $epcc
can have mixed results -- e.g., if CHERI_PERM_LOAD isn't set on $pcc.