runtime: clamp OS-reported number of processors to _MaxGomaxprocs

So that all Go processes do not die on startup on a system with >256 CPUs.

I tested this by hacking osinit to set ncpu to 1000.

Updates #15131
Fixes #15160

Change-Id: I52e061a0de97be41d684dd8b748fa9087d6f1aef
Reviewed-on: https://go-review.googlesource.com/21599Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Reviewed-on: https://go-review.googlesource.com/22206

src/runtime/proc.go

@@ -443,6 +443,9 @@ func schedinit() {
 
 	sched.lastpoll = uint64(nanotime())
 	procs := int(ncpu)
+	if procs > _MaxGomaxprocs {
+		procs = _MaxGomaxprocs
+	}
 	if n := atoi(gogetenv("GOMAXPROCS")); n > 0 {
 		if n > _MaxGomaxprocs {
 			n = _MaxGomaxprocs