Add more debug flags for GCC based compilers

We can use `#define`-ed macros in GDB with `-g3` flag.

We can run code step by step in GDB with `-O0` flag.

Delete temp variable

mruby-config supports MRUBY_LDFLAGS_BEFORE_LIBS in libmruby.flags.mak

Fix possible segfault when accessing backtrace with MRB_WORD_BOXING.

Fix ensure with yield context on break and return

How to reproduce:

    class A
      def x
        yield
      ensure
        y
      end

      def y
      end
    end

    # Work
    A.new.x do
    end

    # Not work
    # trace:
    # 	[2] /tmp/a.rb:5:in A.x
    # 	[0] /tmp/a.rb:15
    # /tmp/a.rb:5: undefined method 'y' for main (NoMethodError)
    A.new.x do
      break
    end

    # trace:
    # 	[2] /tmp/a.rb:5:in A.call
    # 	[0] /tmp/a.rb:19
    # /tmp/a.rb:5: undefined method 'y' for main (NoMethodError)
    lambda do
      A.new.x do
        return
      end
    end.call

`self` in ensure is broken when yield and break/return are used.

Replace int with mrb_bool in dump_bigendian_p func

`mruby -b` now accepts both big/little endian mrb (compiled binary) files.

`mrbc` generates mrb files in big endian for .mrb files and in native endian
for C files (with -B option specified) by default.  If you are cross compiling,
you need to specify target endian by -e/-E options if it is different from
host endian.

Use setter macro instead of directly setting values

update grammar and preposition for debugger readme

tasks/ruby_ext.rake: Fix build error when file path includes \1 or \2.  ...

mrdb, mirb: Add fflush() so that a external program can read output imme...

Don't use Build.current if possible

mrdb: fix crash when stepping into Proc.call

update stable version number in README

How to reproduce:

```
cat a.rb
Proc.new { 1 }.call

echo step | mrdb a.rb
```

fix infinite loop in String#match(arg) when arg is String

Add String#upto

add String#ljust into mruby-string-ext

fix Segmentation fault on Struct#inspect due to recursive Struct object

Use sizeof() instead of strlen().

Properly implement directory tasks