Add FramADR macro
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.