linker: simpler encoding for SHT_RELR sections.

This change modifies the encoding used in SHT_RELR sections to a simpler
version that gives better results. This encoding was suggested by Andrew
Grieve and is described in this post on generic-abi@googlegroups.com:
    https://groups.google.com/d/msg/generic-abi/bX460iggiKg/Pi9aSwwABgAJ

Bug: None
Test: Built image for marlin, flashed on device, ran arm and
      aarch64 binaries containing '.relr.dyn' sections using
      the new encoding.

Change-Id: I266affe0fbad91dc375995985a221cb02499447b