jit: port libgccjit to Windows

2020-05-28  Nicolas Bértolo  <nicolasbertolo@gmail.com>

/ChangeLog:
	* configure.ac: Don't require --enable-host-shared when building
	for Mingw.
	* configure: Regenerate.

2020-05-28  Nicolas Bértolo  <nicolasbertolo@gmail.com>

gcc/ChangeLog:
	* Makefile.in: don't look for libiberty in the "pic" subdirectory
	when building for Mingw. Add dependency on xgcc with the proper
	extension.

2020-05-28  Nicolas Bértolo  <nicolasbertolo@gmail.com>

gcc/c/ChangeLog:
	* Make-lang.in: Remove extra slash.

2020-05-28  Nicolas Bértolo  <nicolasbertolo@gmail.com>

gcc/jit/ChangeLog:
	* Make-lang.in: Remove extra slash. Build libgccjit.dll and its
	import library in Windows.
	* config-lang.in: Update comment about --enable-host-shared.
	* jit-w32.h: New file.
	* jit-w32.c: New file.
	(print_last_error): New function that prints the error
	string corresponding to GetLastError().
	(get_TOKEN_USER_current_user): Helper function used for getting
	the SID belonging to the current user.
	(create_directory_for_current_user): Helper function to create
	a directory with permissions such that only the current user can
	access it.
	(win_mkdtemp): Create a temporary directory using Windows APIs.
	* jit-playback.c: Do not chmod files in Windows. Use LoadLibrary,
	FreeLibrary and GetProcAddress instead of libdl.
	* jit-result.h, jit-result.c: Introduce result::handle_t to
	abstract over the types used for dynamic library handles.
	* jit-tempdir.c: Do not use mkdtemp() in Windows, use
	win_mkdtemp().