x86-64: Add --no-undefined to vDSO build

This gives much nicer diagnostics when something goes wrong.  It's
supported at least as far back as binutils 2.15.
Signed-off-by: Andy Lutomirski <luto@mit.edu>
Link: http://lkml.kernel.org/r/de0b50920469ff6359c529526e7639fdd36fa83c.1310563276.git.luto@mit.eduSigned-off-by: H. Peter Anvin <hpa@linux.intel.com>

arch/x86/vdso/Makefile

@@ -26,6 +26,7 @@ targets += vdso.so vdso.so.dbg vdso.lds $(vobjs-y)
 export CPPFLAGS_vdso.lds += -P -C
 
 VDSO_LDFLAGS_vdso.lds = -m64 -Wl,-soname=linux-vdso.so.1 \
+			-Wl,--no-undefined \
 		      	-Wl,-z,max-page-size=4096 -Wl,-z,common-page-size=4096
 
 $(obj)/vdso.o: $(src)/vdso.S $(obj)/vdso.so