win32.h: define strtoll and strtoull for earlier VC

* include/ruby/win32.h (strtoll, strtoull): VC8 and later until
  VC12 have LONG_LONG but it is _int64, and provide i64 version
  functions only.

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@44875 b2dd03c8-39d4-4d8f-98ff-823fe69b080e