KVM: x86 emulator: simplify ALU opcode block decode further

The ALU opcode block is very regular; introduce D6ALU() to define decode
flags for 6 instructions at a time.

Suggested by Paolo Bonzini.

Signed-off-by: Avi Kivity <avi@redhat.com>
Signed-off-by: Marcelo Tosatti <mtosatti@redhat.com>