load.c: defer static linked ini

* load.c (register_init_ext, ruby_init_ext): defer running the
  registered initialization function until required, not to enable
  extensions which have global effects just by loading, e.g.,
  mathn/complex and mathn/rational.  fix `make test` with
  --with-static-linked-ext.

missing log at r48699t [ci skip]

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@48701 b2dd03c8-39d4-4d8f-98ff-823fe69b080e