[ELF] - Implement SHT_SYMTAB_SHNDX (.symtab_shndxr) section.

This is relative to https://bugs.llvm.org//show_bug.cgi?id=38119.

SHT_SYMTAB section is able to keep symbols with output section indices
up to 0xff00 (SHN_LORESERVE). But if we have indices that are greater
than that (PR shows that it might happen), we need to use
SHT_SYMTAB_SHNDX extended section. It was not supported by LLD.

Description of the SHT_SYMTAB_SHNDX section is here:
https://docs.oracle.com/cd/E19683-01/817-3677/chapter6-94076/index.html.

Differential revision: https://reviews.llvm.org/D49541

llvm-svn: 338247