[ELF] - add support for relocations against local symbols when producing relocatable output.

There was a known limitation for -r option:
relocations against local symbols were not supported. 
For example rel[a].eh_frame sections contained relocations against sections
and that was not supported for -r before. Patch fixes that.

Differential review: http://reviews.llvm.org/D17813

llvm-svn: 262590