X86 isn't the only architecture supporting NUMA_BALANCING. ARM64, PPC,
S390 and RISCV also support it:

arch$ git grep NUMA_BALANCING
arm64/Kconfig:  select ARCH_SUPPORTS_NUMA_BALANCING
arm64/configs/defconfig:CONFIG_NUMA_BALANCING=y
arm64/include/asm/pgtable.h:#ifdef CONFIG_NUMA_BALANCING
powerpc/configs/powernv_defconfig:CONFIG_NUMA_BALANCING=y
powerpc/configs/ppc64_defconfig:CONFIG_NUMA_BALANCING=y
powerpc/configs/pseries_defconfig:CONFIG_NUMA_BALANCING=y
powerpc/include/asm/book3s/64/pgtable.h:#ifdef CONFIG_NUMA_BALANCING
powerpc/include/asm/book3s/64/pgtable.h:#ifdef CONFIG_NUMA_BALANCING
powerpc/include/asm/book3s/64/pgtable.h:#endif /* CONFIG_NUMA_BALANCING */
powerpc/include/asm/book3s/64/pgtable.h:#ifdef CONFIG_NUMA_BALANCING
powerpc/include/asm/book3s/64/pgtable.h:#endif /* CONFIG_NUMA_BALANCING */
powerpc/include/asm/nohash/pgtable.h:#ifdef CONFIG_NUMA_BALANCING
powerpc/include/asm/nohash/pgtable.h:#endif /* CONFIG_NUMA_BALANCING */
powerpc/platforms/Kconfig.cputype:      select ARCH_SUPPORTS_NUMA_BALANCING
riscv/Kconfig:  select ARCH_SUPPORTS_NUMA_BALANCING
riscv/include/asm/pgtable.h:#ifdef CONFIG_NUMA_BALANCING
s390/Kconfig:   select ARCH_SUPPORTS_NUMA_BALANCING
s390/configs/debug_defconfig:CONFIG_NUMA_BALANCING=y
s390/configs/defconfig:CONFIG_NUMA_BALANCING=y
s390/include/asm/pgtable.h:#ifdef CONFIG_NUMA_BALANCING
x86/Kconfig:    select ARCH_SUPPORTS_NUMA_BALANCING     if X86_64
x86/include/asm/pgtable.h:#ifdef CONFIG_NUMA_BALANCING
x86/include/asm/pgtable.h:#endif /* CONFIG_NUMA_BALANCING */

On the other hand, setup_numabalancing() is implemented in mm/mempolicy.c
which doesn't depend on architectures.
Signed-off-by: Barry Song <song.bao.hua@hisilicon.com>
Reviewed-by: Palmer Dabbelt <palmerdabbelt@google.com>
Acked-by: Palmer Dabbelt <palmerdabbelt@google.com>
Cc: Mel Gorman <mgorman@suse.de>
Cc: Paul Walmsley <paul.walmsley@sifive.com>
Cc: Palmer Dabbelt <palmer@dabbelt.com>
Cc: Albert Ou <aou@eecs.berkeley.edu>
Cc: "Paul E. McKenney" <paulmck@kernel.org>
Cc: Randy Dunlap <rdunlap@infradead.org>
Cc: Andrew Morton <akpm@linux-foundation.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>
Cc: Viresh Kumar <viresh.kumar@linaro.org>
Cc: Mike Kravetz <mike.kravetz@oracle.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Link: https://lore.kernel.org/r/20210302084159.33688-1-song.bao.hua@hisilicon.comSigned-off-by: Jonathan Corbet <corbet@lwn.net>