lib9: add getcallerpc.c (fixes build)

src/lib9/getcallerpc.c

@@ -0,0 +1,13 @@
+#include <u.h>
+#include <libc.h>
+
+/*
+ * On gcc and clang, getcallerpc is a macro invoking a compiler builtin.
+ * If the macro in libc.h did not trigger, there's no implementation.
+ */
+#undef getcallerpc
+ulong
+getcallerpc(void *v)
+{
+	return 1;
+}