new_dynarec: Fix MULT 64bits (still wrong maybe).

This fixes Doom 64 in mupen64plus-ae.  It also fixes some (but not all)
collision detection bugs in Donkey Kong 64.

See
https://github.com/ptitSeb/mupen64plus-pandora/commit/ce68e3b9f89132315d1bd69800e364da96340eb8
https://github.com/mupen64plus-ae/mupen64plus-ae/commit/040b15fe91e7d4d351eba4768bcdb79ba8eeb371