Add support for .note.gnu.property sections.

Add support for .note.gnu.property sections.

elfcpp/
	PR gold/22914
	* elfcpp.h (NT_GNU_PROPERTY_TYPE_0): New note type.
	(GNU_PROPERTY_*): New Gnu property types.
	* x86_64.h (GNU_PROPERTY_X86_FEATURE_1_IBT)
	(GNU_PROPERTY_X86_FEATURE_1_SHSTK): New x86 feature bits.

gold/
	PR gold/22914
	* layout.cc (Layout::Layout): Initialize gnu_properties_.
	(read_sized_value, write_sized_value): New functions.
	(Layout::layout_gnu_property): New method.
	(Layout::create_notes): Call create_gnu_properties_note.
	(Layout::create_gnu_properties_note): New method.
	* layout.h (Layout::layout_gnu_property): New method.
	(Layout::create_gnu_properties_note): New method.
	(Layout::Gnu_property, Layout::Gnu_properties): New types.
	(Layout::gnu_properties_): New data member.
	* object.cc (Sized_relobj_file::layout_gnu_property_section): New
	method.
	(Sized_relobj_file::do_layout): Handle .note.gnu.property sections.
	* object.h (Sized_relobj_file::layout_gnu_property_section): New
	method.
	* target.h (Target::merge_gnu_property): New method.
	(Target::do_merge_gnu_property): New virtual method.
	* x86_64.cc (Target_x86_64::do_merge_gnu_property): New method.
	* testsuite/Makefile.am (gnu_property_test): New test case.
	* testsuite/Makefile.in: Regenerate.
	* testsuite/gnu_property_a.S: New source file.
	* testsuite/gnu_property_b.S: New source file.
	* testsuite/gnu_property_c.S: New source file.
	* testsuite/gnu_property_main.c: New source file.
	* testsuite/gnu_property_test.sh: New test script.