Use index of first global symbol to speed up dwfl_module_addrsym search.

Use index of first global symbol to speed up dwfl_module_addrsym search.

Cache the index of the first global symbol in the module's symbol table,
or -1 when unknown.  All symbols with local binding come first in the
symbol table before the global symbols.  Use this information to speed
up dwfl_module_addrsym search.