x86/entry: Correct reference to intended CONFIG_64_BIT

Commit in Fixes adds a condition with IS_ENABLED(CONFIG_64_BIT),
but the intended config item is called CONFIG_64BIT, as defined in
arch/x86/Kconfig.

Fortunately, scripts/checkkconfigsymbols.py warns:

64_BIT
Referencing files: arch/x86/include/asm/entry-common.h

Correct the reference to the intended config symbol.

Fixes: 662a0221 ("x86/entry: Fix AC assertion")
Suggested-by: Randy Dunlap <rdunlap@infradead.org>
Signed-off-by: Lukas Bulwahn <lukas.bulwahn@gmail.com>
Signed-off-by: Borislav Petkov <bp@suse.de>
Cc: <stable@vger.kernel.org>
Link: https://lkml.kernel.org/r/20210803113531.30720-2-lukas.bulwahn@gmail.com