gdb: select "Cygwin" OS ABI for Cygwin binaries

Before this patch, the "Windows" OS ABI is selected for all Windows
executables, including Cygwin ones.  This patch makes GDB differentiate
Cygwin binaries from non-Cygwin ones, and selects the "Cygwin" OS ABI
for the Cygwin ones.

To check whether a Windows PE executable is a Cygwin one, we check the
library list in the .idata section, see if it contains "cygwin1.dll".

I had to add code to parse the .idata section, because BFD doesn't seem
to expose this information.  BFD does parse this information, but only
to print it in textual form (function pe_print_idata):

  https://sourceware.org/git/gitweb.cgi?p=binutils-gdb.git;a=blob;f=bfd/peXXigen.c;h=e42d646552a0ca1e856e082256cd3d943b54ddf0;hb=HEAD#l1261

Here's the relevant portion of the PE format documentation:

  https://docs.microsoft.com/en-us/windows/win32/debug/pe-format#the-idata-section

This page was also useful:

  https://blog.kowalczyk.info/articles/pefileformat.html#9ccef823-67e7-4372-9172-045d7b1fb006

With this patch applied, this is what I get:

    (gdb) file some_mingw_x86_64_binary.exe
    Reading symbols from some_mingw_x86_64_binary.exe...
    (gdb) show osabi
    The current OS ABI is "auto" (currently "Windows").
    The default OS ABI is "GNU/Linux".

    (gdb) file some_mingw_i386_binary.exe
    Reading symbols from some_mingw_i386_binary.exe...
    (gdb) show osabi
    The current OS ABI is "auto" (currently "Windows").
    The default OS ABI is "GNU/Linux".

    (gdb) file some_cygwin_x86_64_binary.exe
    Reading symbols from some_cygwin_x86_64_binary.exe...
    (gdb) show osabi
    The current OS ABI is "auto" (currently "Cygwin").
    The default OS ABI is "GNU/Linux".

gdb/ChangeLog:

	* windows-tdep.h (is_linked_with_cygwin_dll): New declaration.
	* windows-tdep.c (CYGWIN_DLL_NAME): New.
	(pe_import_directory_entry): New struct type.
	(is_linked_with_cygwin_dll): New function.
	* amd64-windows-tdep.c (amd64_windows_osabi_sniffer): Select
	GDB_OSABI_CYGWIN if the BFD is linked with the Cygwin DLL.
	* i386-windows-tdep.c (i386_windows_osabi_sniffer): Likewise.