Improve readelf's selection of a file start symbol when displaying a gnu build attribute.

Improve readelf's selection of a file start symbol when displaying a gnu build attribute.

	* readelf.c (get_symbol_for_build_attribute): Skip ARM mapping
	symbols.
	(print_gnu_build_attribute_description): If no file start symbol
	could be found, look for one two bytes into the file.
	* testsuite/binutils-all/note-4-64.s: Set the address of the file
	start symbol to two bytes into the file.
	* testsuite/binutils-all/note-4-32.s: Likewise.