stack-protector: Fix test with 32-bit userland and CONFIG_64BIT=y

When building a 64-bit 4.18-rc1 kernel with a 32-bit userland, I
noticed that stack protection was silently disabled.  Adding -m64 in
gcc-x86_64-has-stack-protector.sh fixed that, similar to what has been
noticed in commit 2a61f474 ("stack-protector: test compiler
capability in Kconfig and drop AUTO mode") for
gcc-x86_32-has-stack-protector.sh.
Signed-off-by: Sven Joachim <svenjoac@gmx.de>
Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>

scripts/gcc-x86_64-has-stack-protector.sh

 #!/bin/sh
 # SPDX-License-Identifier: GPL-2.0
 
-echo "int foo(void) { char X[200]; return 3; }" | $* -S -x c -c -O0 -mcmodel=kernel -fno-PIE -fstack-protector - -o - 2> /dev/null | grep -q "%gs"
+echo "int foo(void) { char X[200]; return 3; }" | $* -S -x c -c -m64 -O0 -mcmodel=kernel -fno-PIE -fstack-protector - -o - 2> /dev/null | grep -q "%gs"