bignum.c: select by preprocessor conditions

* bignum.c (maxpow_in_bdigit_dbl, maxpow_in_bdigit): select by
  preprocessor conditions to reduce dead code and suppress warnings.

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@41888 b2dd03c8-39d4-4d8f-98ff-823fe69b080e