ruby.h: define RBIGNUM_EMBED_LEN_MAX by macros

* include/ruby/ruby.h (RBIGNUM_EMBED_LEN_MAX): define by macros
  defined in defines.h, instead of complex and repeated expression.

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@43821 b2dd03c8-39d4-4d8f-98ff-823fe69b080e