Some instructions are only available on the 64-bit architecture.

Bi-arch compilers that default to -m32 need the explicit -m64 option
to evaluate them properly.

Fixes: 18e66b69 ("x86/shstk: Add Kconfig option for shadow stack")
Closes: https://lore.kernel.org/all/20240612-as-instr-opt-wrussq-v2-1-bd950f7eead7@gmail.com/Reported-by: Dmitry Safonov <0x7f454c46@gmail.com>
Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>
Signed-off-by: Borislav Petkov (AMD) <bp@alien8.de>
Tested-by: Dmitry Safonov <0x7f454c46@gmail.com>
Link: https://lore.kernel.org/r/20240612050257.3670768-1-masahiroy@kernel.org