gdb: adjust gdbarch_tdep calls in nat files

gdb: adjust gdbarch_tdep calls in nat files

Commit 345bd07cce33 ("gdb: fix gdbarch_tdep ODR violation") forgot to
update the gdbarch_tdep calls in the native files other than x86-64
Linux.  This patch updates them all (to the best of my knowledge).
These are the files I was able to build-test:

  aarch64-linux-nat.c
  amd64-bsd-nat.c
  arm-linux-nat.c
  ppc-linux-nat.c
  windows-nat.c
  xtensa-linux-nat.c

And these are the ones I could not build-test:

  aix-thread.c
  arm-netbsd-nat.c
  ppc-fbsd-nat.c
  ppc-netbsd-nat.c
  ia64-tdep.c (the part that needs libunwind)
  ppc-obsd-nat.c
  rs6000-nat.c

If there are still some build problems related to gdbarch_tdep in them,
they should be pretty obvious to fix.

Change-Id: Iaa3d791a850e4432973757598e634e3da6061428