config: fix i386 configs
    
    Since mainline commit f467c5640c29 ("kconfig: only write '# CONFIG_FOO is
    not set' for visible symbols"), make silentoldconfig (or make syncconfig as
    it is called now) doesn't add CONFIG_64BIT line into i386 configs if called
    with ARCH=i386.
    
    During the build, it's called without ARCH=i386 so that it requests this
    option to be entered manually and the build fails. Long term solution would
    probably be passing ARCH=%{cpu_arch} to all make commands; for now, just
    add CONFIG_64BIT line to i386/pae to fix the build as is.