runtime: bump markStackSize

Every time we overflow the stack, we have to do a full rescan of the heap.  Making this larger
means fewer overflows and thus fewer secondary+ heap scans.

builder/sizes_test.go

@@ -41,7 +41,7 @@ func TestBinarySize(t *testing.T) {
 	// This is a small number of very diverse targets that we want to test.
 	tests := []sizeTest{
 		// microcontrollers
-		{"hifive1b", "examples/echo", 4560, 280, 0, 2268},
+		{"hifive1b", "examples/echo", 4568, 280, 0, 2268},
 		{"microbit", "examples/serial", 2868, 388, 8, 2272},
 		{"wioterminal", "examples/pininterrupt", 6104, 1484, 116, 6832},
 

src/runtime/gc_blocks.go

@@ -45,7 +45,7 @@ const (
 	bytesPerBlock      = wordsPerBlock * unsafe.Sizeof(heapStart)
 	stateBits          = 2 // how many bits a block state takes (see blockState type)
 	blocksPerStateByte = 8 / stateBits
-	markStackSize      = 4 * unsafe.Sizeof((*int)(nil)) // number of to-be-marked blocks to queue before forcing a rescan
+	markStackSize      = 8 * unsafe.Sizeof((*int)(nil)) // number of to-be-marked blocks to queue before forcing a rescan
 )
 
 var (