Implement CGo support

This mostly works around the default CGo support: this compiler doesn't
need special wrappers for C functions as it uses the C calling
convention by default.

src/runtime/cgo/cgo.go

@@ -0,0 +1,4 @@
+
+package cgo
+
+// dummy

src/syscall/syscall.go

@@ -0,0 +1,6 @@
+
+package syscall
+
+// dummy
+
+type Errno uintptr

src/unsafe/unsafe.go

@@ -0,0 +1,4 @@
+
+package unsafe
+
+// dummy