Add definition for ELF dynamic tag DT_SYMTAB_SHNDX.

DT_SYMTAB_SHNDX is defined in generic-abi:

http://www.sco.com/developers/gabi/latest/ch5.dynamic.html

Patch by Rahul Chaudhry!

Differential Revision: https://reviews.llvm.org/D47803

llvm-svn: 334149