runtime: eliminate GOMAXPROCS limit

Now that allp is dynamically allocated, there's no need for a hard cap
on GOMAXPROCS.

Fixes #15131.

Change-Id: I53eee8e228a711a818f7ebce8d9fd915b3865eed
Reviewed-on: https://go-review.googlesource.com/45574
Run-TryBot: Austin Clements <austin@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Rick Hudson <rlh@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

Author: aclements

src/runtime/debug.go

@@ -15,9 +15,6 @@ import (
 // The number of logical CPUs on the local machine can be queried with NumCPU.
 // This call will go away when the scheduler improves.
 func GOMAXPROCS(n int) int {
-	if n > _MaxGomaxprocs {
-		n = _MaxGomaxprocs
-	}
 	lock(&sched.lock)
 	ret := int(gomaxprocs)
 	unlock(&sched.lock)

src/runtime/proc.go

@@ -495,9 +495,6 @@ func schedinit() {
 	if n, ok := atoi32(gogetenv("GOMAXPROCS")); ok && n > 0 {
 		procs = n
 	}
-	if procs > _MaxGomaxprocs {
-		procs = _MaxGomaxprocs
-	}
 	if procresize(procs) != nil {
 		throw("unknown runnable goroutine during bootstrap")
 	}
@@ -3529,7 +3526,7 @@ func setcpuprofilerate(hz int32) {
 // Returns list of Ps with local work, they need to be scheduled by the caller.
 func procresize(nprocs int32) *p {
 	old := gomaxprocs
-	if old < 0 || old > _MaxGomaxprocs || nprocs <= 0 || nprocs > _MaxGomaxprocs {
+	if old < 0 || nprocs <= 0 {
 		throw("procresize: invalid arg")
 	}
 	if trace.enabled {

src/runtime/runtime2.go

@@ -520,12 +520,6 @@ type p struct {
 	pad [sys.CacheLineSize]byte
 }
 
-const (
-	// The max value of GOMAXPROCS.
-	// There are no fundamental restrictions on the value.
-	_MaxGomaxprocs = 1 << 10
-)
-
 type schedt struct {
 	// accessed atomically. keep at top to ensure alignment on 32-bit systems.
 	goidgen  uint64