s390/vdso: Drop '-shared' from KBUILD_CFLAGS_64

When clang's -Qunused-arguments is dropped from KBUILD_CPPFLAGS, it
points out that there is a linking phase flag added to CFLAGS, which
will only be used for compiling

  clang-16: error: argument unused during compilation: '-shared' [-Werror,-Wunused-command-line-argument]

'-shared' is already present in ldflags-y so it can just be dropped.

Fixes: 2b2a258 ("s390/vdso: Use $(LD) instead of $(CC) to link vDSO")
Signed-off-by: Nathan Chancellor <nathan@kernel.org>
Acked-by: Heiko Carstens <hca@linux.ibm.com>
Reviewed-by: Sven Schnelle <svens@linux.ibm.com>
Tested-by: Linux Kernel Functional Testing <lkft@linaro.org>
Tested-by: Anders Roxell <anders.roxell@linaro.org>
Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>

Author: nathanchance

arch/s390/kernel/vdso64/Makefile

@@ -25,7 +25,7 @@ KBUILD_AFLAGS_64 := $(filter-out -m64,$(KBUILD_AFLAGS))
 KBUILD_AFLAGS_64 += -m64
 
 KBUILD_CFLAGS_64 := $(filter-out -m64,$(KBUILD_CFLAGS))
-KBUILD_CFLAGS_64 += -m64 -fPIC -shared -fno-common -fno-builtin
+KBUILD_CFLAGS_64 += -m64 -fPIC -fno-common -fno-builtin
 ldflags-y := -fPIC -shared -soname=linux-vdso64.so.1 \
 	     --hash-style=both --build-id=sha1 -T