Skip PT_DYNAMIC segment with p_filesz == 0 [BZ #22101]

ELF objects generated with "objcopy --only-keep-debug" have

Type     Offset  VirtAddr   PhysAddr   FileSiz MemSiz  Flg Align
DYNAMIC  0x0+e28 0x0+200e40 0x0+200e40 0x0+    0x0+1a0 RW  0x8

with 0 file size. ld.so should skip such PT_DYNAMIC segments.

Without a PT_DYNAMIC segment the loading of the shared object will
fail, and therefore ldd on such objects will also fail instead of
crashing. This provides better diagnostics for tooling that is
attempting to inspect the invalid shared objects which may just
contain debug information.

	[BZ #22101]
	* elf/Makefile (tests): Add tst-debug1.
	($(objpfx)tst-debug1): New.
	($(objpfx)tst-debug1.out): Likewise.
	($(objpfx)tst-debug1mod1.so): Likewise.
	* elf/dl-load.c (_dl_map_object_from_fd): Skip PT_DYNAMIC segment
	with p_filesz == 0.
	* elf/tst-debug1.c: New file.

Reviewed-by: Carlos O'Donell <carlos@redhat.com>