attributes for XKPHYS.
o) Make coprocessor 0 accessor function macros for register+selector registers
take the full name so that e.g. (as done in this commit), prid selector 1
can be written through mips_wr_ebase() rather than mips_wr_prid1().
o) Allow for sign extension of 32-bit segment addresses.
o) Remove an unused MIPS-I register number.