GCC 6: i386: Move struct ix86_frame to machine_function

[H.J. Lu]

This is needed for GCC 6 backport of Spectre patches:

https://gcc.gnu.org/ml/gcc-patches/2018-01/msg01465.html
https://gcc.gnu.org/ml/gcc-patches/2018-01/msg01466.html
https://gcc.gnu.org/ml/gcc-patches/2018-01/msg01464.html

-- 
H.J.