sys/cdefs.h: fix for use __restrict in C++

Newlib shares large parts of <sys/cdefs.h> with FreeBSD and received
this bug report:

https://sourceware.org/pipermail/newlib/2023/020400.html

As an extension, GCC and clang offer C99-style restricted pointers in
C++ mode:
https://gcc.gnu.org/onlinedocs/gcc/Restricted-Pointers.html

We notice that this extension is broken when including newlib headers:
restricted pointers are treated as ordinary pointers.

We traced this to the following section of
newlib/libc/include/sys/cdefs.h:

/*
 * GCC 2.95 provides `__restrict' as an extension to C90 to support the
 * C99-specific `restrict' type qualifier.  We happen to use `__restrict' as
 * a way to define the `restrict' type qualifier without disturbing older
 * software that is unaware of C99 keywords.
 */
#if !(__GNUC__ == 2 && __GNUC_MINOR__ == 95)
#if !defined(__STDC_VERSION__) || __STDC_VERSION__ < 199901
#define	__restrict
#else
#define	__restrict	restrict
#endif
#endif

While the GCC __restrict extension was indeed introduced in GCC 2.95, it
is not limited to this version; the extension is also not limited to
C90:
https://gcc.gnu.org/gcc-2.95/c++features.html

Rewrite the logic in the header so that __restrict is kept alone when
available.

PR: 272723
MFC after: 1 week