Indicate the location pointed to by '$ra'

[ralfgerlich]

While 'JAL' and the 'BGEZAL'/'BLTZAL' instructions use '$ra' as the hard-coded link register, the value of the '$ra' register itself is not shown.

We could add it to the register tabs, but there's nowhere it would properly fit conceptually. As it typically contains an instruction location, indicating the return location in the editor - as is currently done with the next instruction to run and the last instruction executed - seems to be a pretty user-friendly solution.