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Updates the image builder to support non-relocatable ELF images and
instantiates the tests for the non-relocatable images.

Replaces GetElfBaseVirtualAddress() with GetRelocationOffset().
Relocation is a well defined concept in the ELF standard, while
'base virtual address' is not, and is at odds with the 'base address'
concept.  For clarity, avoids the implicit assumption that the first
load segment's file offset is 0.

Makes the handling of the ELF base address consistent among the
functions.

Bug: 1101023
Change-Id: I6eac949a79a02e7e3a662f604193bf414d5ad38f
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2305570
Commit-Queue: Mike Wittman <wittman@chromium.org>
Reviewed-by: Wez <wez@chromium.org>
Reviewed-by: Kevin Marshall <kmarshall@chromium.org>
Cr-Commit-Position: refs/heads/master@{#794659}