runtime: pass a smaller buffer to sched_getaffinity on ARM

The system stack is only around 8kb on ARM so one can't put an 8kb buffer on
the stack. More than 1024 ARM cores seems sufficiently unlikely for the
foreseeable future.

Fixes #11853

Change-Id: I7cb27c1250a6153f86e269c172054e9dfc218c72
Reviewed-on: https://go-review.googlesource.com/12622
Reviewed-by: Austin Clements <austin@google.com>

src/runtime/os1_linux.go

@@ -76,13 +76,14 @@ func futexwakeup(addr *uint32, cnt uint32) {
 
 func getproccount() int32 {
 	// This buffer is huge (8 kB) but we are on the system stack
-	// and there should be plenty of space (64 kB).
+	// and there should be plenty of space (64 kB) -- except on ARM where
+	// the system stack itself is only 8kb (see golang.org/issue/11873).
 	// Also this is a leaf, so we're not holding up the memory for long.
 	// See golang.org/issue/11823.
 	// The suggested behavior here is to keep trying with ever-larger
 	// buffers, but we don't have a dynamic memory allocator at the
 	// moment, so that's a bit tricky and seems like overkill.
-	const maxCPUs = 64 * 1024
+	const maxCPUs = 64*1024*(1-goarch_arm) + 1024*goarch_arm
 	var buf [maxCPUs / (ptrSize * 8)]uintptr
 	r := sched_getaffinity(0, unsafe.Sizeof(buf), &buf[0])
 	n := int32(0)