[llvm-readelf]Don't lose negative-ness of negative addends for no symbol relocations
llvm-readelf prints relocation addends as:
<symbol value>[+-]<absolute addend>
where [+-] is determined from whether addend is less than zero or not.
However, it does not print the +/- if there is no symbol, which meant
that negative addends became their positive value with no indication
that this had happened. This patch stops the absolute conversion when
addends are negative and there is no associated symbol.
Reviewed by: Higuoxing, mattd, MaskRay
Differential Revision: https://reviews.llvm.org/D59095
llvm-svn: 355696