const the core magic vtables

const the core magic vtables

A remark in
http://www.nntp.perl.org/group/perl.perl5.porters/2006/07/msg115113.html
caused commit bc028b6b7f /
http://www.nntp.perl.org/group/perl.perl5.porters/2006/07/msg115116.html .
That remark is not true according to
http://www.nntp.perl.org/group/perl.perl5.porters/2006/07/msg115115.html
and
http://www.nntp.perl.org/group/perl.perl5.porters/2013/01/msg197318.html .
To save memory between perl processes, const the tables so the memory is
shared by the OS between perl processes.

b4 .rdata section of x64 VC 2008 miniperl 0x667EE bytes .data 0x3440 bytes
after .rdata 0x66FAE .data 0x2C80, 0x3440-0x2C80=0x7C0 saved,
based on the .data section size crossing a page boundary this saved 4K
of per process memory on Win64 miniperl.