compile.c: fix RB_GC_GUARD usage

* compile.c (iseq_build_from_ary_exception): move RB_GC_GUARD
  (iseq_build_from_ary_body): use PRIsVALUE instead of RB_GC_GUARD

Currently unused functions, but we should set good examples for
using RB_GC_GUARD (and favoring PRIsVALUE in format strings).

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@47881 b2dd03c8-39d4-4d8f-98ff-823fe69b080e