[llvm-readobj] Derive dynamic symtab size from DT_HASH

If the section headers have been removed by a tool such as llvm-objcopy
or llvm-strip, previously llvm-readobj/llvm-readelf would not dump the
dynamic symbols when --dyn-symbols was specified. However, the nchain
value of the DT_HASH data specifies the number of dynamic symbols, so if
it is present, we can use that. This patch implements this behaviour.

Fixes https://bugs.llvm.org/show_bug.cgi?id=45089.

Reviewed by: grimar, MaskRay

Differential Revision: https://reviews.llvm.org/D76352