compiler: define atomic intrinsic functions directly

This changes the compiler from treating calls to sync/atomic.* functions
as special calls (emitted directly at the call site) to actually
defining their declarations when there is no Go SSA implementation. And
rely on the inliner to inline these very small functions.
This works a bit better in practice. For example, this makes it possible
to use these functions in deferred function calls.

This commit is a bit large because it also needs to refactor a few
things to make it possible to define such intrinsic functions.

testdata/atomic.go

@@ -81,6 +81,9 @@ func main() {
 	// test atomic.Value load/store operations
 	testValue(int(3), int(-2))
 	testValue("", "foobar", "baz")
+
+	// Test atomic operations as deferred values.
+	testDefer()
 }
 
 func testValue(values ...interface{}) {
@@ -93,3 +96,11 @@ func testValue(values ...interface{}) {
 		}
 	}
 }
+
+func testDefer() {
+	n1 := int32(5)
+	defer func() {
+		println("deferred atomic add:", n1)
+	}()
+	defer atomic.AddInt32(&n1, 3)
+}

testdata/atomic.txt

@@ -33,3 +33,4 @@ StoreUint32: 20
 StoreUint64: 20
 StoreUintptr: 20
 StorePointer: true
+deferred atomic add: 8