all: change special type __volatile to pragma //go:volatile

This is one step towards removing unnecessary special casts in most
cases. It is also part of removing as much magic as possible from the
compiler (the pragma is explicit, the special name is not).

src/device/arm/arm.go

@@ -33,8 +33,8 @@ import (
 	"unsafe"
 )
 
-type __volatile uint32
-type RegValue = __volatile
+//go:volatile
+type RegValue uint32
 
 type __asm string
 
@@ -47,7 +47,7 @@ const (
 
 // Nested Vectored Interrupt Controller (NVIC).
 type NVIC_Type struct {
-	ISER [8]__volatile
+	ISER [8]RegValue
 }
 
 var NVIC = (*NVIC_Type)(unsafe.Pointer(uintptr(NVIC_BASE)))