* vm_exec.c (cfp): Avoid generating invalid binary for
  NativeClient.
  r15 on x86_64 is reserved by NativeClient.  So r15 to cfp used to
  generate invalid binary under some combinations of compiler
  optimization flags.

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@44554 b2dd03c8-39d4-4d8f-98ff-823fe69b080e