powerpc: select CPUMASK_OFFSTACK if NR_CPUS >= 8192

Some core kernel code starts to go beyond the 2048 byte stack size
warning at NR_CPUS=8192, so select CPUMASK_OFFSTACK in that case.
x86 does similarly for very large NR_CPUS.
Signed-off-by: Nicholas Piggin <npiggin@gmail.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/20211105035042.1398309-2-npiggin@gmail.com

arch/powerpc/Kconfig

@@ -165,6 +165,7 @@ config PPC
 	select BINFMT_ELF
 	select BUILDTIME_TABLE_SORT
 	select CLONE_BACKWARDS
+	select CPUMASK_OFFSTACK			if NR_CPUS >= 8192
 	select DCACHE_WORD_ACCESS		if PPC64 && CPU_LITTLE_ENDIAN
 	select DMA_OPS_BYPASS			if PPC64
 	select DMA_OPS				if PPC64