stage2 x86_64: fix comptime integer multiplication when rhs=0
stage2 x86_64: fix comptime integer multiplication when rhs=0

Co-authored-by: joachimschmidt557 <5c744fdfb17bd1ed962f6eb9f7498a16e59a7c44@outlook.com>

andrewrk/zig,zig-lang/zig,zig-lang/zig,zig-lang/zig,andrewrk/zig