ARC: mm: fix build failure in linux-next for UP builds

kisskb build service reported ARC defconfig build failures in linux-next

| arch/arc/include/asm/mmu.h:75:21: error: 'NR_CPUS' undeclared here (not in a function)
| make[3]: *** [arch/arc/mm/ioremap.o] Error 1
| make[2]: *** [arch/arc/mm] Error 2
| make[1]: *** [arch/arc] Error 2

which I bisected to a subtle side-effect of a totally benign mm patch
("mm, vmalloc: properly track vmalloc users") which caused a header
include chain deviation - asm/mmu.h using NR_CPUS before including
linux/threads.h

Fix that by adding the dependnet header and while it at fix a related
header to include linux headers aheads of asm headers as sometimes that
slso triggers such issues !

Reported-by:  <noreply@ellerman.id.au>
Signed-off-by: Vineet Gupta <vgupta@synopsys.com>