NAT_FILE set incorrectly in gdb/Makefile

[Eli Zaretskii]

"make TAGS" curses thusly:

  Making TAGS
  .././gdb/config/i386/config/i386/nm-linux64.h: No such file or directory

This is on GNU/Linux, obviously, but I see the equivalent error
message in the DJGPP build.

The problem is this line from gdb/Makefile:

  NAT_FILE= config/i386/config/i386/nm-linux64.h

which is obviously incorrect.  It looks like the culprit is this line
from configure.ac:

  sed -e '/^NAT_FILE[     ]*=/s,^NAT_FILE[        ]*=[    ]*,&config/'"${gdb_host_cpu}"'/,' <Makefile >Makefile.tmp

which seems to run twice, perhaps once by configure, the other time by
config.status.  But that's a guess; the truth is I have no idea why
config/i386 gets prepended twice.  Does anyone see the villain?