re-implement mrb_float_to_str()

The new implementation is backwards incompatible, but I couldn't find
any usage outside mruby and I also couldn't think of a different and
good name.

All ISO C99 printf conversion specifiers for floating point numbers and
an optional precision are supported.

It is largely based on code from the MIT licensed musl libc
(http://www.musl-libc.org/) and its floating point printing is exact
(unlike the current code behind Float#to_s).

Make backtraces work again under DISABLE_STDIO

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