compiler: implement volatile operations as compiler builtins

The long term goal is to remove the //go:volatile hack.

src/runtime/volatile/volatile.go

@@ -0,0 +1,34 @@
+// Package volatile provides definitions for volatile loads and stores. These
+// are implemented as compiler builtins.
+//
+// The load operations load a volatile value. The store operations store to a
+// volatile value. The compiler will emit exactly one load or store operation
+// when possible and will not reorder volatile operations. However, the compiler
+// may move other operations across load/store operations, so make sure that all
+// relevant loads/stores are done in a volatile way if this is a problem.
+//
+// These loads and stores are commonly used to read/write values from memory
+// mapped peripheral devices. They do not provide atomicity, use the sync/atomic
+// package for that.
+//
+// For more details: https://llvm.org/docs/LangRef.html#volatile-memory-accesses
+// and https://blog.regehr.org/archives/28.
+package volatile
+
+// LoadUint8 loads the volatile value *addr.
+func LoadUint8(addr *uint8) (val uint8)
+
+// LoadUint16 loads the volatile value *addr.
+func LoadUint16(addr *uint16) (val uint16)
+
+// LoadUint32 loads the volatile value *addr.
+func LoadUint32(addr *uint32) (val uint32)
+
+// StoreUint8 stores val to the volatile value *addr.
+func StoreUint8(addr *uint8, val uint8)
+
+// StoreUint16 stores val to the volatile value *addr.
+func StoreUint16(addr *uint16, val uint16)
+
+// StoreUint32 stores val to the volatile value *addr.
+func StoreUint32(addr *uint32, val uint32)