um: vdso: Always reject undefined references in during linking

Instead of using a custom script to detect and fail on undefined
references, use --no-undefined for all VDSO linker invocations.

Drop the now unused checkundef.sh script.

Signed-off-by: Thomas Weißschuh <thomas.weissschuh@linutronix.de>
Link: https://patch.msgid.link/20241011-vdso-checkundef-v1-2-1a46e0352d20@linutronix.de
Signed-off-by: Johannes Berg <johannes.berg@intel.com>

Author: t-8ch

arch/x86/um/vdso/Makefile

@@ -56,7 +56,6 @@ CFLAGS_REMOVE_um_vdso.o = -pg -fprofile-arcs -ftest-coverage
 quiet_cmd_vdso = VDSO    $@
       cmd_vdso = $(CC) -nostdlib -o $@ \
 		       $(CC_FLAGS_LTO) $(VDSO_LDFLAGS) $(VDSO_LDFLAGS_$(filter %.lds,$(^F))) \
-		       -Wl,-T,$(filter %.lds,$^) $(filter %.o,$^) && \
-		 sh $(src)/checkundef.sh '$(NM)' '$@'
+		       -Wl,-T,$(filter %.lds,$^) $(filter %.o,$^)
 
-VDSO_LDFLAGS = -fPIC -shared -Wl,--hash-style=sysv -z noexecstack
+VDSO_LDFLAGS = -fPIC -shared -Wl,--hash-style=sysv -z noexecstack -Wl,--no-undefined