gdb: add Windows OS ABI

GDB currently uses the "Cygwin" OS ABI (GDB_OSABI_CYGWIN) for everything
related to Windows.  If you build a GDB for a MinGW or Cygwin target, it
will have "Cygwin" as the default OS ABI in both cases (see
configure.tgt).  If you load either a MinGW or Cygwin binary, the
"Cygwin" OS ABI will be selected in both cases.

This is misleading, because Cygwin binaries are a subset of the binaries
running on Windows.  When building something with MinGW, the resulting
binary has nothing to do with Cygwin.  Cygwin binaries are only special
in that they are Windows binaries that link to the cygwin1.dll library
(if my understanding is correct).

Looking at i386-cygwin-tdep.c, we can see that GDB does nothing
different when dealing with Cygwin binaries versus non-Cygwin Windows
binaries.  However, there is at least one known bug which would require
us to make a distinction between the two OS ABIs, and that is the size
of the built-in "long" type on x86-64.  On native Windows, this is 4,
whereas on Cygwin it's 8.

So, this patch adds a new OS ABI, "Windows", and makes GDB use it for
i386 and x86-64 PE executables, instead of the "Cygwin" OS ABI.  A
subsequent patch will improve the OS ABI detection so that GDB
differentiates the non-Cygwin Windows binaries from the Cygwin Windows
binaries, and applies the "Cygwin" OS ABI for the latter.

The default OS ABI remains "Cygwin" for the GDBs built with a Cygwin
target.

I've decided to split the i386_cygwin_osabi_sniffer function in two,
I think it's cleaner to have a separate sniffer for Windows binaries and
Cygwin cores, each checking one specific thing.

gdb/ChangeLog:

	* osabi.h (enum gdb_osabi): Add GDB_OSABI_WINDOWS.
	* osabi.c (gdb_osabi_names): Add "Windows".
	* i386-cygwin-tdep.c (i386_cygwin_osabi_sniffer): Return
	GDB_OSABI_WINDOWS when the binary's target is "pei-i386".
	(i386_cygwin_core_osabi_sniffer): New function, extracted from
	i386_cygwin_osabi_sniffer.
	(_initialize_i386_cygwin_tdep): Register OS ABI
	GDB_OSABI_WINDOWS for i386.
	* amd64-windows-tdep.c (amd64_windows_osabi_sniffer): Return
	GDB_OSABI_WINDOWS when the binary's target is "pei-x86-64".
	(_initialize_amd64_windows_tdep): Register OS ABI GDB_OSABI_WINDOWS
	for x86-64.
	* configure.tgt: Use GDB_OSABI_WINDOWS as the default OS ABI
	when the target matches '*-*-mingw*'.

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