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>