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)
```

1 files changed, 2 insertions, 0 deletions

diff --git a/Rakefile b/Rakefile
index caabd8bac..503fbc5c5 100644
--- a/Rakefile
+++ b/Rakefile
@@ -195,6 +195,7 @@ file presym_file => cfiles+rbfiles+psfiles+[__FILE__] do
   presyms = File.readlines(presym_file) rescue []
   presyms.each{|x| x.chomp!}
   if presyms != symbols
+    _pp "GEN", presym_file.relative_path
     File.write(presym_file, symbols.join("\n"))
     Rake::Task[presym_inc].invoke
   end
@@ -207,6 +208,7 @@ task presym_inc do
   prefix_re = Regexp.union(*symbol_to_macro.keys.uniq.map(&:first))
   suffix_re = Regexp.union(*symbol_to_macro.keys.uniq.map(&:last))
   macro_re = /\A(#{prefix_re})?([\w&&\D]\w*)(#{suffix_re})?\z/o
+  _pp "GEN", presym_inc.relative_path
   File.open(presym_inc, "w") do |f|
     f.puts "/* MRB_PRESYM_NAMED(lit, num, type, name) */"
     f.puts "/* MRB_PRESYM_UNNAMED(lit, num) */"