Fix i386 and x86-64 generic multiplication hooks on fixnums.

Fix i386 and x86-64 generic multiplication hooks on fixnums.

Before multiplying, both tags must be cleared, and one of the
operands must be shifted to include a factor of 2^6.  Shifting both
operands, so that there is an extra factor of 2^12, or not clearing
the tag on both operands, is wrong.  However, this didn't affect any
real code, since the fixnum case of generic multiplication is always
open-coded by the compiler unless you futz with compiler switches.