SHT_SYMTAB_SHNDX handling

SHT_SYMTAB_SHNDX handling

.symtab_shndx section contents is an array, one entry for each symbol
in .symtab, present when the number of symbols exceeds a little less
than 64k.  Since the mapping is 1-1 with symbols there is no need to
keep both dest_index and destshndx_index in elf_sym_strtab.  Instead,
just make sure that the shndx pointers to the swap functions are kept
NULL when .symtab_shndx does not exist.  Also, strtabcount in the
linker's elf hash table is incremented in lock-step with the output
symcount, so that can disappear too.