gc.c: rb_gc_set_params

* gc.c (rb_gc_set_params): define as separate function.
  RUBY_ALIAS_FUNCTION is for simple alias only, the third parameter
  will be ignored on gcc.

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@43993 b2dd03c8-39d4-4d8f-98ff-823fe69b080e