S/390: Rename source files to *-linux-*

S/390: Rename source files to *-linux-*

As suggested before, rename the S/390-related source files (tdep and nat)
such that "-linux-" occurs in the file name, like with other GNU/Linux
targets.  Since no other operating system is currently supported by GDB
on this architecture, this isn't strictly necessary.  But the old names
sometimes caused GDB contributors to miss these files when performing a
change that affects all GNU/Linux targets.  The latest such incident was
observed here:

    https://sourceware.org/ml/gdb-patches/2013-09/msg00619.html

gdb/
2013-10-30  Andreas Arnez  <arnez@linux.vnet.ibm.com>

	* s390-tdep.h: Rename to...
	* s390-linux-tdep.h: ...here.
	* s390-tdep.c: Rename to...
	* s390-linux-tdep.c: ...here.  Adjust #include.
	* s390-nat.c: Rename to...
	* s390-linux-nat.c: ...here.  Adjust #include.
	* config/s390/s390.mh: Rename to...
	* config/s390/linux.mh: ...here.  Reflect rename s390-nat.o ->
	s390-linux-nat.o.
	* configure.host: Reflect host rename "s390" -> "linux".
	* configure.tgt: Reflect rename s390-tdep.o -> s390-linux-tdep.o.
	* Makefile.in (ALL_TARGET_OBS): Likewise.
	(HFILES_NO_SRCDIR): Reflect rename s390-tdep.h ->
	s390-linux-tdep.h.
	(ALLDEPFILES): Reflect rename of .c files.