Switch back full path to pocl.icd. Due to popular demand.

Add notes about this for packagers in README.packaging

README.packaging

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+This file contains notes for making distribution packages of pocl.
+
+ICD  
+--- 
+Pocl should probably be built with ICD enabled for desktop
+distributions. Pocl does not have an ICD loader, so a dependancy
+on one would be beneficial.
+
+The pocl.icd file (which the ICD loader uses to load the pocl lib)
+has a full path to the installed libpocl.so file. When building
+for a multiarch, remove the "@libdir@/" from pocl.icd.in before
+building. This way the system library loader can pick up the correct
+architecture library.