kbuild: Fix '-S -c' in x86 stack protector scripts

After a recent change in clang to stop consuming all instances of '-S'
and '-c' [1], the stack protector scripts break due to the kernel's use
of -Werror=unused-command-line-argument to catch cases where flags are
not being properly consumed by the compiler driver:

  $ echo | clang -o - -x c - -S -c -Werror=unused-command-line-argument
  clang: error: argument unused during compilation: '-c' [-Werror,-Wunused-command-line-argument]

This results in CONFIG_STACKPROTECTOR getting disabled because
CONFIG_CC_HAS_SANE_STACKPROTECTOR is no longer set.

'-c' and '-S' both instruct the compiler to stop at different stages of
the pipeline ('-S' after compiling, '-c' after assembling), so having
them present together in the same command makes little sense. In this
case, the test wants to stop before assembling because it is looking at
the textual assembly output of the compiler for either '%fs' or '%gs',
so remove '-c' from the list of arguments to resolve the error.

All versions of GCC continue to work after this change, along with
versions of clang that do or do not contain the change mentioned above.

Cc: stable@vger.kernel.org
Fixes: 4f7fd4d7 ("[PATCH] Add the -fstack-protector option to the CFLAGS")
Fixes: 60a5317f ("x86: implement x86_32 stack protector")
Link: https://github.com/llvm/llvm-project/commit/6461e537815f7fa68cef06842505353cf5600e9c

 [1]
Signed-off-by: Nathan Chancellor <nathan@kernel.org>
Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>