string.c (rb_str_format_m): trade volatile for RB_GC_GUARD

* string.c (rb_str_format_m): trade volatile for RB_GC_GUARD
  RB_GC_GUARD meaning is clear and has better code generation.
  [ruby-core:60688]

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@44951 b2dd03c8-39d4-4d8f-98ff-823fe69b080e