[MIPS] Update struct sigcontext member names

    
Rename the 64-bit sc_hi and sc_lo arrays to use the same names
as the 32-bit struct sigcontext (sc_mdhi, sc_hi1, et cetera).
    
Signed-off-by: Daniel Jacobowitz <dan@codesourcery.com>
Signed-off-by: Ralf Baechle <ralf@linux-mips.org>