s390/purgatory: Remove unused '-MD' and unnecessary '-c' flags

When clang's -Qunused-arguments is dropped from KBUILD_CPPFLAGS, it
warns while building objects in the purgatory folder:

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

'-MMD' is always passed to the preprocessor via c_flags, even when
KBUILD_CFLAGS is overridden in a folder, so clang complains the addition
of '-MD' will be unused. Remove '-MD' to clear up this warning, as it is
unnecessary with '-MMD'.

Additionally, '-c' is also unnecessary, remove it while in the area.
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>

arch/s390/purgatory/Makefile

@@ -24,7 +24,7 @@ KCSAN_SANITIZE := n
 KBUILD_CFLAGS := -fno-strict-aliasing -Wall -Wstrict-prototypes
 KBUILD_CFLAGS += -Wno-pointer-sign -Wno-sign-compare
 KBUILD_CFLAGS += -fno-zero-initialized-in-bss -fno-builtin -ffreestanding
-KBUILD_CFLAGS += -c -MD -Os -m64 -msoft-float -fno-common
+KBUILD_CFLAGS += -Os -m64 -msoft-float -fno-common
 KBUILD_CFLAGS += -fno-stack-protector
 KBUILD_CFLAGS += $(CLANG_FLAGS)
 KBUILD_CFLAGS += $(call cc-option,-fno-PIE)