No PT_INTERP when .interp is zero size

No PT_INTERP when .interp is zero size

Some targets don't set a default interpreter, resulting in an empty
.interp section unless --dynamic-linker is passed to ld.  A PT_INTERP
without a path is rather useless.

The testsuite change fixes a failure on microblaze-linux.

bfd/
	* elf.c (get_program_header_size): Don't count PT_INTERP if
	.interp is empty.
	(_bfd_elf_map_sections_to_segments): Don't create PT_INTERP if
	.interp is empty.
ld/
	* testsuite/ld-elf/pr22423.d: Pass --dynamic-linker to ld.