DynASM/x86: Add shld/shrd opcodes.

Thanks to Peter Cawley.
Merge branch 'master' into v2.1
DynASM/x86: Add fldenv, f[n]stenv, fxsave, fxrstor opcodes.

Thanks to Cosmin Apreutesei.
Merge branch 'master' into v2.1
Merge branch 'v2.1' into v2.1-agentzh