runtime: on 32-bit systems, limit default GOMAXPROCS to 32

Otherwise we can easily run out of stack space for threads.

The user can still override by setting GOMAXPROCS.

Reviewed-on: https://go-review.googlesource.com/c/gofrontend/+/219278

gcc/go/gofrontend/MERGE

-c94637ad6fd38d4814fb02d094a1a73f19323d71
+3e46519cee5c916a9b39480fbac13f4ffc6a93b0
 
 The first line of this file holds the git revision number of the last
 merge done from the gofrontend repository.

libgo/go/runtime/proc.go

@@ -563,6 +563,14 @@ func schedinit() {
 
 	sched.lastpoll = uint64(nanotime())
 	procs := ncpu
+
+	// In 32-bit mode, we can burn a lot of memory on thread stacks.
+	// Try to avoid this by limiting the number of threads we run
+	// by default.
+	if sys.PtrSize == 4 && procs > 32 {
+		procs = 32
+	}
+
 	if n, ok := atoi32(gogetenv("GOMAXPROCS")); ok && n > 0 {
 		procs = n
 	}