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While poking around in FileSwitch I spotted that (a) it currently isn't
using the FramLDR/FramSTR macros (instead manually calculating offsets
from Proc_LocalStack), and (b) in a few places it was using
`ADD dest, sp, #Proc_LocalStack+n*4` to get the address of an argument.

Add a FramADR macro which can be used to perform that task, so that
everything is ready for switching FileSwitch over to using the Fram
macros.

Version 3.02. Tagged as 'HdrSrc-3_02'