reorder MY_CXT_CLONE for less memory reads

reorder MY_CXT_CLONE for less memory reads

Nn VC 2003 32, taking a very simple CLONE XSUB, specifically
Time::HiRes::CLONE, shows a reduction from 0x53 to 0x47 bytes of machine
code. This is because my_cxt_index has to be reread after the memcpy
function call incase my_cxt_index was changed by memcpy (GCC usually
inlines short fixed length memcpys, on VC P5P perl, the option to inline
memcpy is off), also the new my_cxtp does not need to be saved in a non-vol
anymore, previously my_cxtp had to be copied to a non-vol for it be
available after the memcpy function call. In a simple XSUB like the one
mentioned here, saving and restoring the non-vol register is also
removed.

See details in perl #123534.