Fix writedocs.

This probably got broken by aosp/1609473 because soong_build is now
invoked in two different ways:

- By its absolute path during building out/soong/build.ninja
- By its path relative to the source root when generating documentation

Bug: 182347000
Test: Manual (the long incantation on b/182347000)
Change-Id: I0a3a97e4865ffde4a450a3f7baf3980b5a2016c8

android/writedocs.go

@@ -35,10 +35,11 @@ type docsSingleton struct{}
 
 func primaryBuilderPath(ctx SingletonContext) Path {
 	buildDir := absolutePath(ctx.Config().BuildDir())
-	primaryBuilder, err := filepath.Rel(buildDir, os.Args[0])
+	binary := absolutePath(os.Args[0])
+	primaryBuilder, err := filepath.Rel(buildDir, binary)
 	if err != nil {
-		ctx.Errorf("path to primary builder %q is not in build dir %q",
-			os.Args[0], ctx.Config().BuildDir())
+		ctx.Errorf("path to primary builder %q is not in build dir %q (%q)",
+			os.Args[0], ctx.Config().BuildDir(), err)
 	}
 
 	return PathForOutput(ctx, primaryBuilder)