implementation of locore routines between MIPS1 and MIPS3. It's
independent from mips_locore_jumpvec_t which is for cache/TLB
manipulating routines peculiar to processor designs. mips_locore_jumpvec_t
will be replaced with "processor closures" encapsulating implementation
parameters (cpuinfo) and pointers to conventaion routines (cpuops),
eventually.