gdb: Move DJGPP/go32 bits to their own tdep file

gdb: Move DJGPP/go32 bits to their own tdep file

I posit that this makes them easier to find.

The other day while working on the wchar_t patch, I had a bit of
trouble finding the DJGPP/go32 tdep bits.  My initial reaction was
looking for a go32-specific tdep file, but there's none.

Confirmed that a --host=i586-pc-msdosdjgpp GDB still builds
successfully and includes the  i386-go32-tdep.o object.

Confirmed that an --enable-targets=all build of GDB on x86-64
GNU/Linux includes the DJGPP/go32 bits too.

gdb/ChangeLog:
2017-04-12  Pedro Alves  <palves@redhat.com>

	* Makefile.in (ALL_TARGET_OBS): Add i386-go32-tdep.o.
	* configure.tgt: Handle i[34567]86-*-go32* and
	i[34567]86-*-msdosdjgpp*.
	* i386-tdep.c (i386_svr4_reg_to_regnum):
	Make extern.
	(i386_go32_init_abi, i386_coff_osabi_sniffer): Moved to
	i386-go32-tdep.c.
	(_initialize_i386_tdep): DJGPP bits moved to i386-go32-tdep.c.
	* i386-go32-tdep.c: New file.
	* i386-tdep.h (tdesc_i386_mmx, i386_svr4_reg_to_regnum): New
	declarations.