x86/build: Make 64-bit defconfig the default

Running 'make ARCH=x86 defconfig' on anything other than an x86_64
machine currently results in a 32-bit build, which is rarely what
anyone wants these days.

Change the default so that the 64-bit config gets used unless
the user asks for i386_defconfig, uses ARCH=i386 or runs on
a system that "uname -m" identifies as i386/i486/i586/i686.

Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Ingo Molnar <mingo@kernel.org>
Link: https://lore.kernel.org/r/20230215091706.1623070-1-arnd@kernel.org