[Patch,AVR] Support instrucions XCH, LAT, LAC, LAS

[nick clifton]

Hi Georg-Johann,

> (AVR_ISA_XCH): New define.
> 	(AVR_ISA_XMEGA): Use it.
> 	(XCH, LAS, LAT, LAC): New XMEGA opcodes.

Approved and applied.  (Sorry for the delay)

Cheers
   Nick