Add PHX/PLX and PHY/PLY

[BigEd]

These are quite difficult to replace with macros and very useful to preserve registers.

They are defined in 65C02

[BigEd]

Arlet implementation notes from http://forum.6502.org/viewtopic.php?p=15834#p15834:

In the first place, the opcode mask needs to include the new opcode here
http://github.com/Arlet/verilog-6502/blob/c51aa17b6b1a4de23184815fd404b4670186f8ae/cpu.v#L877

Secondly, it needs an extra opcode mask to set src_reg to SEL_X register here
https://github.com/Arlet/verilog-6502/blob/c51aa17b6b1a4de23184815fd404b4670186f8ae/cpu.v#L1016

Of course, with all opcode extensions you have to watch out that the new opcode doesn't overlap with a "don't care" in one of the existing patterns.