internal/gclayout: make gclayout values constants

The previous versions calculated at init() prevented `interp` from running
in many cases, increasing compile times due to the increased need to revert
the partially interpreted results and also increasing binary runtime because
fewer optimizations had happened during interp.

src/runtime/gc_boehm.go

@@ -89,7 +89,7 @@ func alloc(size uintptr, layout unsafe.Pointer) unsafe.Pointer {
 	gcLock.Lock()
 	needsResumeWorld = false
 	var ptr unsafe.Pointer
-	if layout == gclayout.NoPtrs {
+	if layout == gclayout.NoPtrs.AsPtr() {
 		// This object is entirely pointer free, for example make([]int, ...).
 		// Make sure the GC knows this so it doesn't scan the object
 		// unnecessarily to improve performance.

src/runtime/slice.go

@@ -51,7 +51,7 @@ func sliceGrow(oldBuf unsafe.Pointer, oldLen, oldCap, newCap, elemSize uintptr)
 	var layout unsafe.Pointer
 	// less type info here; can only go off element size
 	if elemSize < unsafe.Sizeof(uintptr(0)) {
-		layout = gclayout.NoPtrs
+		layout = gclayout.NoPtrs.AsPtr()
 	}
 
 	buf := alloc(newCap*elemSize, layout)

src/runtime/string.go

@@ -60,7 +60,7 @@ func stringConcat(x, y _string) _string {
 		return x
 	} else {
 		length := x.length + y.length
-		buf := alloc(length, gclayout.NoPtrs)
+		buf := alloc(length, gclayout.NoPtrs.AsPtr())
 		memcpy(buf, unsafe.Pointer(x.ptr), x.length)
 		memcpy(unsafe.Add(buf, x.length), unsafe.Pointer(y.ptr), y.length)
 		return _string{ptr: (*byte)(buf), length: length}
@@ -73,7 +73,7 @@ func stringFromBytes(x struct {
 	len uintptr
 	cap uintptr
 }) _string {
-	buf := alloc(x.len, gclayout.NoPtrs)
+	buf := alloc(x.len, gclayout.NoPtrs.AsPtr())
 	memcpy(buf, unsafe.Pointer(x.ptr), x.len)
 	return _string{ptr: (*byte)(buf), length: x.len}
 }
@@ -84,7 +84,7 @@ func stringToBytes(x _string) (slice struct {
 	len uintptr
 	cap uintptr
 }) {
-	buf := alloc(x.length, gclayout.NoPtrs)
+	buf := alloc(x.length, gclayout.NoPtrs.AsPtr())
 	memcpy(buf, unsafe.Pointer(x.ptr), x.length)
 	slice.ptr = (*byte)(buf)
 	slice.len = x.length
@@ -101,7 +101,7 @@ func stringFromRunes(runeSlice []rune) (s _string) {
 	}
 
 	// Allocate memory for the string.
-	s.ptr = (*byte)(alloc(s.length, gclayout.NoPtrs))
+	s.ptr = (*byte)(alloc(s.length, gclayout.NoPtrs.AsPtr()))
 
 	// Encode runes to UTF-8 and store the resulting bytes in the string.
 	index := uintptr(0)