[llvm-objcopy/strip] Fix off-by-one error in SYMTAB_SHNDX need check

The check for whether an extended symbol index table was required
dropped the first SHN_LORESERVE sections from the sections array before
checking whether the remaining sections had symbols. Unfortunately, the
null section header is not present in this list, so the check was
skipping the first section that might be important. If that section
contained a symbol, and no subsequent ones did, the .symtab_shndx
section would not be emitted, leading to a corrupt object.

Also consolidate and expand test coverage in the area to cover this bug
and other aspects of the SYMTAB_SHNDX section.

Reviewed by: alexshap, MaskRay

Differential Revision: https://reviews.llvm.org/D97661