Don't write uninitialised data in .note.gnu.property

Seen when running the binutils x86-64 pr23494 tests.
_bfd_elf_convert_gnu_properties doesn't initialise its "contents"
buffer, and elf_write_gnu_properties doesn't write to padding.
Fix this by initialising the padding and then dispense with zeroing
contents in _bfd_elf_link_setup_gnu_properties.

	* elf-properties.c (elf_write_gnu_properties): Init padding.
	(_bfd_elf_link_setup_gnu_properties): Don't init contents.