Re-code the saving of the gp register for MIPS64 so it compiles

In working on a project that wants to use greenlet on a MIPS64-based Linux system, I ran into the following compilation error:

    In file included from slp_platformselect.h:36:0,
                      from greenlet.c:416:
    platform/switch_mips_unix.h: In function 'slp_switch':
    platform/switch_mips_unix.h:26:5: error: PIC register clobbered by '$28' in 'asm'
         __asm__ __volatile__ ("" : : : REGS_TO_SAVE);
         ^
    platform/switch_mips_unix.h:41:5: error: PIC register clobbered by '$28' in 'asm'
         __asm__ __volatile__ ("" : : : REGS_TO_SAVE);
         ^

This was using gcc 4.7 and 4.9.  To fix this, I came up with this alternative way of saving the gp register.  The generated assembly looks correct, and all tests are passing.