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_cortexm.go

@@ -26,7 +26,7 @@ func preinit() {
 	ptr := unsafe.Pointer(&_sbss)
 	for ptr != unsafe.Pointer(&_ebss) {
 		*(*uint32)(ptr) = 0
-		ptr = unsafe.Pointer(uintptr(ptr) + 4)
+		ptr = unsafe.Add(ptr, 4)
 	}
 
 	// Initialize .data: global variables initialized from flash.
@@ -34,8 +34,8 @@ func preinit() {
 	dst := unsafe.Pointer(&_sdata)
 	for dst != unsafe.Pointer(&_edata) {
 		*(*uint32)(dst) = *(*uint32)(src)
-		dst = unsafe.Pointer(uintptr(dst) + 4)
-		src = unsafe.Pointer(uintptr(src) + 4)
+		dst = unsafe.Add(dst, 4)
+		src = unsafe.Add(src, 4)
 	}
 }