[ELF] Add .gnu.linkonce support.

When the GNU linker sees two input sections with the same name, and the name
starts with ".gnu.linkonce.", the linker will only keep one copy and discard the
other. Any section whose name starts with “.gnu.linkonce.” is a COMDAT section.

Some architectures like Hexagon use this section to store floating point constants,
that need be deduped.

This patch adds gnu.linkonce functionality to the ELFReader.

llvm-svn: 230194