iostream: Properly support the "x" mode for SDL_IOFromFile()

The "x" mode for `fopen()` (open file only if it doesn't exist) used to
be a glibc-exclusive extension, but was later standardized in C11, and
is now also implemented as part of every other widely-used libc:

	* musl: https://git.musl-libc.org/cgit/musl/tree/src/stdio/__fmodeflags.c?id=0ccaf0572e9cccda2cced0f7ee659af4c1c6679a
	* Android Bionic / OpenBSD: 731631f300/libc/upstream-openbsd/lib/libc/stdio/flags.c (86)
	* Apple / FreeBSD: 63976b830a/stdio/FreeBSD/flags.c (L91-L92)

As a result, "x" has already been working on all our automatically
tested platforms that implement `SDL_IOFromFile()` via `fopen()`. So
all we'd be missing for proper support is a Windows implementation
using `CREATE_NEW`, and the documentation that this mode exists and is
intended to work.

include/SDL3/SDL_iostream.h

@@ -203,6 +203,8 @@ typedef struct SDL_IOStream SDL_IOStream;
  * - "w": Create an empty file for writing. If a file with the same name
  *   already exists its content is erased and the file is treated as a new
  *   empty file.
+ * - "wx": Create an empty file for writing. If a file with the same name
+ *   already exists, the call fails.
  * - "a": Append to a file. Writing operations append data at the end of the
  *   file. The file is created if it does not exist.
  * - "r+": Open a file for update both reading and writing. The file must
@@ -210,6 +212,8 @@ typedef struct SDL_IOStream SDL_IOStream;
  * - "w+": Create an empty file for both reading and writing. If a file with
  *   the same name already exists its content is erased and the file is
  *   treated as a new empty file.
+ * - "w+x": Create an empty file for both reading and writing. If a file with
+ *    the same name already exists, the call fails.
  * - "a+": Open a file for reading and appending. All writing operations are
  *   performed at the end of the file, protecting the previous content to be
  *   overwritten. You can reposition (fseek, rewind) the internal pointer to