Revert: r249728 - Roll back r249726 and r249723 because they broke buildbots.

Reapply r249726 (and r249723), hopefully with the correct test fixups this time.

Original commit message:

Address a FIXME in ELF/Writer.cpp: Make VAStart a target-dependent property.
I've set the values for the existing targets to what I believe to be the
correct values, and updated the regression tests accordingly.

llvm-svn: 249752