doc: make doc testable

doc/_readme/llgo_call_c/call_c.go

@@ -0,0 +1,10 @@
+package main
+
+import _ "unsafe" // for go:linkname
+
+//go:linkname Sqrt C.sqrt
+func Sqrt(x float64) float64
+
+func main() {
+	println("sqrt(2) =", Sqrt(2))
+}