x86: Drop support for /proc files when !CONFIG_PROC_FS

arch/x86/kernel/cpu/proc.c only exists to support files in /proc; omit that
file when compiling without CONFIG_PROC_FS.

Saves 645 additional bytes on 32-bit x86 when !CONFIG_PROC_FS:

add/remove: 0/5 grow/shrink: 0/0 up/down: 0/-645 (-645)
function                                     old     new   delta
c_stop                                         1       -      -1
c_next                                        11       -     -11
cpuinfo_op                                    16       -     -16
c_start                                       24       -     -24
show_cpuinfo                                 593       -    -593

Signed-off-by: Josh Triplett <josh@joshtriplett.org>