all: use unsafe.Add instead of unsafe.Pointer(uintptr(...) + ...)

We have an optimization for this specific pattern, but it's really just
a hack. With the addition of unsafe.Add in Go 1.17 we can directly
specify the intent instead and eventually remove this special case.

The code is also easier to read.

src/runtime/runtime.go

@@ -57,8 +57,8 @@ func malloc(size uintptr) unsafe.Pointer
 // Compare two same-size buffers for equality.
 func memequal(x, y unsafe.Pointer, n uintptr) bool {
 	for i := uintptr(0); i < n; i++ {
-		cx := *(*uint8)(unsafe.Pointer(uintptr(x) + i))
-		cy := *(*uint8)(unsafe.Pointer(uintptr(y) + i))
+		cx := *(*uint8)(unsafe.Add(x, i))
+		cy := *(*uint8)(unsafe.Add(y, i))
 		if cx != cy {
 			return false
 		}