gdb: recognize 64 bits Windows executables as Cygwin osabi

gdb: recognize 64 bits Windows executables as Cygwin osabi

If I generate two Windows PE executables, one 32 bits and one 64 bits:

    $ x86_64-w64-mingw32-gcc test.c -g3 -O0 -o test_64
    $ i686-w64-mingw32-gcc test.c -g3 -O0 -o test_32
    $ file test_64
    test_64: PE32+ executable (console) x86-64, for MS Windows
    $ file test_32
    test_32: PE32 executable (console) Intel 80386, for MS Windows

When I load the 32 bits binary in my GNU/Linux-hosted GDB, the osabi is
correctly recognized as "Cygwin":

    $ ./gdb --data-directory=data-directory -nx test_32
    (gdb) show osabi
    The current OS ABI is "auto" (currently "Cygwin").

When I load the 64 bits binary in GDB, the osabi is incorrectly
recognized as "GNU/Linux":

    $ ./gdb --data-directory=data-directory -nx test_64
    (gdb) show osabi
    The current OS ABI is "auto" (currently "GNU/Linux").

The 32 bits one gets recognized by the i386_cygwin_osabi_sniffer
function, by its target name:

    if (strcmp (target_name, "pei-i386") == 0)
      return GDB_OSABI_CYGWIN;

The target name for the 64 bits binaries is "pei-x86-64".  It doesn't
get recognized by any osabi sniffer, so GDB falls back on its default
osabi, "GNU/Linux".

This patch adds an osabi sniffer function for the Windows 64 bits
executables in amd64-windows-tdep.c.  With it, the osabi is recognized
as "Cygwin", just like with the 32 bits binary.

Note that it may seems strange to have a binary generated by MinGW
(which has nothing to do with Cygwin) be recognized as a Cygwin binary.
This is indeed not accurate, but at the moment GDB uses the Cygwin for
everything Windows.  Subsequent patches will add a separate "Windows" OS
ABI for Windows binaries that are not Cygwin binaries.

gdb/ChangeLog:

	* amd64-windows-tdep.c (amd64_windows_osabi_sniffer): New
	function.
	(_initialize_amd64_windows_tdep): Register osabi sniffer.