ESI is not available as callee save register on linux 64, so use R12, which
is available on windows 64, too. The only callee save register still available on
all platforms is EBP, but i guess we don't want to fiddle with that one.
git-svn-id: https://dolphin-emu.googlecode.com/svn/trunk@6492 8ced0084-cf51-0410-be5f-012b33b47a6e