[ELF] - Fix for: error "invalid section index: xxx" when linking FreeBSD kernel.

We found that GNU assembler 2.17.50 [FreeBSD] 2007-07-03
could generate broken objects. STT_SECTION symbols can be
associated with SHT_REL[A]/SHT_SYMTAB/SHT_STRTAB sections.

This is PR28868, patch fixes handling of such files.

Differential revision: https://reviews.llvm.org/D23201

llvm-svn: 278550