unicode: refactor the rule for regenerating utf8data.h

scripts/mkutf8data is used only when regenerating utf8data.h,
which never happens in the normal kernel build. However, it is
irrespectively built if CONFIG_UNICODE is enabled.

Moreover, there is no good reason for it to reside in the scripts/
directory since it is only used in fs/unicode/.

Hence, move it from scripts/ to fs/unicode/.

In some cases, we bypass build artifacts in the normal build. The
conventional way to do so is to surround the code with ifdef REGENERATE_*.

For example,

 - 7373f4f8 ("kbuild: add implicit rules for parser generation")
 - 6aaf49b4

 ("crypto: arm,arm64 - Fix random regeneration of S_shipped")

I rewrote the rule in a more kbuild'ish style.

In the normal build, utf8data.h is just shipped from the check-in file.

$ make
  [ snip ]
  SHIPPED fs/unicode/utf8data.h
  CC      fs/unicode/utf8-norm.o
  CC      fs/unicode/utf8-core.o
  CC      fs/unicode/utf8-selftest.o
  AR      fs/unicode/built-in.a

If you want to generate utf8data.h based on UCD, put *.txt files into
fs/unicode/, then pass REGENERATE_UTF8DATA=1 from the command line.
The mkutf8data tool will be automatically compiled to generate the
utf8data.h from the *.txt files.

$ make REGENERATE_UTF8DATA=1
  [ snip ]
  HOSTCC  fs/unicode/mkutf8data
  GEN     fs/unicode/utf8data.h
  CC      fs/unicode/utf8-norm.o
  CC      fs/unicode/utf8-core.o
  CC      fs/unicode/utf8-selftest.o
  AR      fs/unicode/built-in.a

I renamed the check-in utf8data.h to utf8data.h_shipped so that this
will work for the out-of-tree build.

You can update it based on the latest UCD like this:

$ make REGENERATE_UTF8DATA=1 fs/unicode/
$ cp fs/unicode/utf8data.h fs/unicode/utf8data.h_shipped

Also, I added entries to .gitignore and dontdiff.

Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>
Signed-off-by: Theodore Ts'o <tytso@mit.edu>