Create presym files for each build target

Previously, presym files were always created in `build/{presym,presym.inc}`.
However, this constraint is inconvenient because it is common to use
multiple build configurations and build targets in a single mruby tree.
Therefore, change to create presym file for each build target.

lib/mruby/build.rb

@@ -373,6 +373,14 @@ EOS
     def libraries
       [libmruby_static]
     end
+
+    def presym_file
+      "#{build_dir}/presym"
+    end
+
+    def presym_inc
+      "#{build_dir}/include/mruby/presym.inc"
+    end
   end # Build
 
   class CrossBuild < Build