Integrate perlio:

[ 12737]
Tweak for build with Mingw32 - __attribute__((unused)) is not liked
by gcc -xc++.

[ 12736]
Have :stdio layer update an honour the fd refcnt table.
Embed.t now passes with PERLIO=stdio as well (tested under ithreads)

p4raw-link: @12737 on //depot/perlio: 1c85e96a88f6bac005ba2c0b226e3ed81d5ca9cc
p4raw-link: @12736 on //depot/perlio: 1751d01517ae68e043553e12532e976a8feb2f80

p4raw-id: //depot/perl@12738