add const to Perl_boot_core_UNIVERSAL's xsub registration struct

add const to Perl_boot_core_UNIVERSAL's xsub registration struct

Move struct xsub_details details to RO memory from RW memory. This increases
the amount of bytes of the image that can be shared between Perl processes
by the OS. The inverse is each perl process takes less process specific
memory. I saw no change in .text, .rdata went from 0x21121 to 0x21391
(+0x270), .data went from 0x3b18 to 0x38b8 (-0x260). 32 bit Visual C.