Support parsing symbol table in dynamic segment.

Unlike SymbolTableSection, this patch reads from DT_SYMTAB, and
DT_STRTAB in PT_DYNAMIC segment. It heuristically determines the end of
DT_SYMTAB by finding the next higher pointer in the same segment. GNU
libc (dl-addr.c) assumes that DT_STRTAB comes after DT_SYMTAB.