[llvm-readelf] - Do not omit a zero symbol value when printing relocations.

Previously we only printed a symbol value when it has a non-empty name
or non-zero value.

This patch changes the behavior. Now we only omit a symbols value when
a relocation does not reference a symbol (i.e. symbol index == 0).

Seems it is what GNU readelf does, looking on its output.

Differential revision: https://reviews.llvm.org/D81842