x86/defconfigs: Explicitly unset CONFIG_64BIT in i386_defconfig

A recent refresh of the defconfigs got rid of the following
(unset) config:

  # CONFIG_64BIT is not set

Innocuous as it seems, when the config file is saved again the
behavior is changed so that CONFIG_64BIT=y.

Currently,

  $ make i386_defconfig
  $ grep CONFIG_64BIT .config
  CONFIG_64BIT=y

whereas previously (and with this patch):

  $ make i386_defconfig
  $ grep CONFIG_64BIT .config
  # CONFIG_64BIT is not set

( This was found with weird compiler errors on OpenEmbedded
  builds, as the compiler was unable to cope with 64-bits data
  types. )

Fixes: 1d0e12fd

 ("x86/defconfigs: Refresh defconfig files")
Reported-by: Jarkko Nikula <jarkko.nikula@linux.intel.com>
Reported-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Tested-by: Sedat Dilek <sedat.dilek@gmail.com>
Signed-off-by: Daniel Díaz <daniel.diaz@linaro.org>
Signed-off-by: Ingo Molnar <mingo@kernel.org>