i386: Emit correct error code for 64-bit IDT entry

i386: Emit correct error code for 64-bit IDT entry

When in 64-bit mode, IDT entiries are 16 bytes, so `intno * 16` is used
for base/limit/offset calculations. However, even in 64-bit mode, the
exception error code still uses bits [3,16) for the invlaid interrupt
index.

This means the error code should still be `intno * 8 + 2` even in 64-bit
mode.

Resolves: https://gitlab.com/qemu-project/qemu/-/issues/1382
Signed-off-by: Joe Richey <joerichey@google.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>