Allow the GNU gold-style nonstandard SHT_GROUP section.

The ELF standard defines that the SHT_GROUP section as follows:

 - its sh_link has the symbol index, and
 - the symbol name is used to uniquify section groups.

Object files created by GNU gold does not seem to comply with the
standard. They have this additional rule:

 - if the symbol has no name and a STT_SECTION symbol, a section
   name is used instead of a symbol name.

If we don't do anything for this, the linker fails with a mysterious
error message if input files are generated by gas. It is unfortunate
but I think we need to support it.

Differential Revision: https://reviews.llvm.org/D34064

llvm-svn: 305218