Replace remaining references to i386-nat with x86-nat instead.

i386-nat.[hc] got renamed to x86-nat.[hc] a while back, but somehow
3 references to the old file name remained past the renaming. This
fixes all of them.

gdb/ChangeLog (with Mike Stump <mikestump@comcast.net>):

        * Makefile.in (TAGS): Replace i386-nat.h by x86-nat.h.
        * x86-nat.c: Replace remaining references to i386-nat
        by reference to x86-nat instead.