kernel: improve handling of CONFIG_IO_URING

Kernel setting CONFIG_IO_URING supports high-performance I/O for file
access and servers, generally for more performant platforms, and adds
~45 KB to kernel sizes. The need for this on less "beefy" devices is
questionable, as is the size cost considering many platforms have kernel
size limits which require tricky repartitioning if outgrown. The size
cost is also large relative to the ~180 KB bump expected between major
OpenWRT kernel releases.

No OpenWrt packages have hard dependencies on this; samba4 and mariadb
can take advantage if available (+KERNEL_IO_URING:liburing) but
otherwise build and work fine.

Since CONFIG_IO_URING is already managed via the KERNEL_IO_URING setting
in Config-kernel.in (default Y), remove it from those target configs
which unconditionally enable it, and update the defaults to enable it
conditionally only on more powerful 64-bit x86 and arm devices. It may
still be manually enabled as needed for high-performance custom builds.

Signed-off-by: Tony Ambardar <itugrok@yahoo.com>