x86-64, vdso: Fix vDSO build breakage due to empty .rela.dyn

Certain ld versions (observed with 2.20.0) put an empty .rela.dyn
section into shared object files, breaking the assumption on the number
of sections to be copied to the final output. Simply discard any empty
SHT_REL and SHT_RELA sections to address this.

Signed-off-by: Jan Beulich <jbeulich@suse.com>
Link: http://lkml.kernel.org/r/53B5861E02000078000204D1@mail.emea.novell.com
Acked-by: Andy Lutomirski <luto@amacapital.net>
Tested-by: Boris Ostrovsky <boris.ostrovsky@oracle.com>
Tested-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: H. Peter Anvin <hpa@linux.intel.com>

arch/x86/vdso/vdso2c.h

@@ -93,6 +93,9 @@ static void BITSFUNC(copy_section)(struct BITSFUNC(fake_sections) *out,
 	uint64_t flags = GET_LE(&in->sh_flags);
 
 	bool copy = flags & SHF_ALLOC &&
+		(GET_LE(&in->sh_size) ||
+		 (GET_LE(&in->sh_type) != SHT_RELA &&
+		  GET_LE(&in->sh_type) != SHT_REL)) &&
 		strcmp(name, ".altinstructions") &&
 		strcmp(name, ".altinstr_replacement");