Try harder to find the correct symbol to associate with OPEN GNU BUILD notes.

Try harder to find the correct symbol to associate with OPEN GNU BUILD notes.

	* readelf.c (print_gnu_build_attribute_description): Move symbol
	printing code to...
	(print_symbol_for_build_attribute): New function.  ...here.
	Add to find the best symbol to associate with an OPEN note.
	Add code to cache the symbol table and string table, so that they
	are not loaded every time a note is displayed.
	* testsuite/binutils-all/note-2-32.s: Add a function symbol.
	* testsuite/binutils-all/note-2-64.s: Likewise.
	* testsuite/binutils-all/note-2-32.d: Update expected note output.
	* testsuite/binutils-all/note-2-64.d: Likewise.