Refine build log for generated files

* Output `GEN` log for generated files
* `MRBC` log is outputted one for each `mrbc` execution

#### Before this patch:

```console
CC    src/array.c -> build/host/src/array.o
(snip)
GEN   mrblib/*.rb -> build/host/mrblib/mrblib.c
      MRBC mrblib/00class.rb
      MRBC mrblib/10error.rb
(snip)
CC    mrbgems/mruby-time/src/time.c -> build/host/mrbgems/mruby-time/src/time.o
      MRBC mrbgems/mruby-time/mrblib/time.rb
(snip)
CC    mrbgems/mruby-socket/test/sockettest.c -> build/host/mrbgems/mruby-socket/test/sockettest.o
      MRBC mrbgems/mruby-socket/test/addrinfo.rb
      MRBC mrbgems/mruby-socket/test/basicsocket.rb
(snip)
```

#### After this patch:

```console
GEN   build/presym
GEN   build/presym.inc
CC    src/array.c -> build/host/src/array.o
(snip)
GEN   mrblib/*.rb -> build/host/mrblib/mrblib.c
      MRBC mrblib/00class.rb
           mrblib/10error.rb
(snip)
CC    mrbgems/mruby-time/src/time.c -> build/host/mrbgems/mruby-time/src/time.o
GEN   build/host/mrbgems/mruby-time/gem_init.c
      MRBC mrbgems/mruby-time/mrblib/time.rb
(snip)
CC    mrbgems/mruby-socket/test/sockettest.c -> build/host/mrbgems/mruby-socket/test/sockettest.o
GEN   build/host/mrbgems/mruby-socket/gem_test.c
      MRBC mrbgems/mruby-socket/test/addrinfo.rb
      MRBC mrbgems/mruby-socket/test/basicsocket.rb
(snip)
```

lib/mruby/build/command.rb

@@ -308,8 +308,8 @@ module MRuby
     def run(out, infiles, funcname, cdump = true)
       @command ||= @build.mrbcfile
       infiles = [infiles].flatten
-      infiles.each do |f|
-        _pp "MRBC", f.relative_path, nil, :indent => 2
+      infiles.each_with_index do |f, i|
+        _pp i == 0 ? "MRBC" : "", f.relative_path, indent: 2
       end
       cmd = %Q["#{filename @command}" #{cdump ? "-S" : ""} #{@compile_options % {:funcname => funcname}} #{filename(infiles).map{|f| %Q["#{f}"]}.join(' ')}]
       puts cmd if Rake.verbose

lib/mruby/core_ext.rb

@@ -26,10 +26,10 @@ def install_D(src, dst)
   cp src, dst
 end
 
-def _pp(cmd, src, tgt=nil, options={})
+def _pp(cmd, src, tgt=nil, indent: nil)
   return if Rake.verbose
 
   width = 5
-  template = options[:indent] ? "%#{width*options[:indent]}s %s %s" : "%-#{width}s %s %s"
+  template = indent ? "%#{width * indent}s %s %s" : "%-#{width}s %s %s"
   puts template % [cmd, src, tgt ? "-> #{tgt}" : nil]
 end

lib/mruby/gem.rb

@@ -184,6 +184,7 @@ module MRuby
       end
 
       def generate_gem_init(fname)
+        _pp "GEN", fname.relative_path
         open(fname, 'w') do |f|
           print_gem_init_header f
           unless rbfiles.empty?