riscv: vdso: fix and clean-up Makefile

Running "make" on an already compiled kernel tree will rebuild the
kernel even without any modifications:

  CALL    linux/scripts/checksyscalls.sh
  CALL    linux/scripts/atomic/check-atomics.sh
  CHK     include/generated/compile.h
  SO2S    arch/riscv/kernel/vdso/vdso-syms.S
  AS      arch/riscv/kernel/vdso/vdso-syms.o
  AR      arch/riscv/kernel/vdso/built-in.a
  AR      arch/riscv/kernel/built-in.a
  AR      arch/riscv/built-in.a
  GEN     .version
  CHK     include/generated/compile.h
  UPD     include/generated/compile.h
  CC      init/version.o
  AR      init/built-in.a
  LD      vmlinux.o

The reason is "Any target that utilizes if_changed must be listed in
$(targets), otherwise the command line check will fail, and the target
will always be built" as explained by Documentation/kbuild/makefiles.rst

Fix this build bug by adding vdso-syms.S to $(targets)

At the same time, there are two trivial clean up modifications:

- the vdso-dummy.o is not needed any more after so remove it.

- vdso.lds is a generated file, so it should be prefixed with
  $(obj)/ instead of $(src)/

Fixes: c2c81bb2f6 ("RISC-V: Fix the VDSO symbol generaton for binutils-2.35+")
Cc: stable@vger.kernel.org
Signed-off-by: Jisheng Zhang <jszhang@kernel.org>
Signed-off-by: Palmer Dabbelt <palmerdabbelt@google.com>

arch/riscv/kernel/vdso/Makefile

@@ -23,7 +23,7 @@ ifneq ($(c-gettimeofday-y),)
 endif
 
 # Build rules
-targets := $(obj-vdso) vdso.so vdso.so.dbg vdso.lds vdso-dummy.o
+targets := $(obj-vdso) vdso.so vdso.so.dbg vdso.lds vdso-syms.S
 obj-vdso := $(addprefix $(obj)/, $(obj-vdso))
 
 obj-y += vdso.o vdso-syms.o
@@ -41,7 +41,7 @@ KASAN_SANITIZE := n
 $(obj)/vdso.o: $(obj)/vdso.so
 
 # link rule for the .so file, .lds has to be first
-$(obj)/vdso.so.dbg: $(src)/vdso.lds $(obj-vdso) FORCE
+$(obj)/vdso.so.dbg: $(obj)/vdso.lds $(obj-vdso) FORCE
 	$(call if_changed,vdsold)
 LDFLAGS_vdso.so.dbg = -shared -s -soname=linux-vdso.so.1 \
 	--build-id=sha1 --hash-style=both --eh-frame-hdr