[ELF][test]Remove unnecessary empty symbol references in yaml/add missing symbols for relocs

yaml2obj used to require the Symbol field in relocations, but it hasn't
done so for a couple of years. Another change to yaml2obj will soon land
that will look up the symbol by name or index, if present, and emit an
error if not found. This will mean that an explicit symbol reference
(even to an empty-named symbol) that does not reference a symbol
declared in the yaml will result in an error.

This patch updates tests that would otherwise start emitting errors.

Reviewed by: ruiu, grimar

Differential Revision: https://reviews.llvm.org/D58508

llvm-svn: 354666