elf: Strip zero-sized dynamic sections

ELF size_dynamic_sections is called by the ELF backend linker after all
the linker input files have been seen but before the section sizes have
been set.  After the sections sizes have been set, target-specific,
global optimizations may make some dynamic sections zero-sized if they
are no longer needed.

Add ELF strip_zero_sized_dynamic_sections so that ELF backend linker can
strip zero-sized dynamic sections after the sections sizes have been set.

bfd/

	PR ld/25849
	* elf-bfd.h (elf_backend_data): Add
	elf_backend_strip_zero_sized_dynamic_sections.
	(_bfd_elf_strip_zero_sized_dynamic_sections): New prototype.
	* elf64-alpha.c (elf_backend_strip_zero_sized_dynamic_sections):
	New macro.
	* elflink.c (_bfd_elf_strip_zero_sized_dynamic_sections): New
	function.
	* elfxx-target.h (elf_backend_strip_zero_sized_dynamic_sections):
	New macro.
	(elfNN_bed): Add elf_backend_strip_zero_sized_dynamic_sections.

ld/

	PR ld/25849
	* ldelfgen.c (ldelf_map_segments): Call
	elf_backend_strip_zero_sized_dynamic_sections.
	* testsuite/ld-alpha/tlsbinr.rd: Updated.