__restrict defined away for C++

[Jim Wilson <jimw@sifive.com>]

This problem was originally reported on the RISC-V sw-dev mailing list.
https://groups.google.com/a/groups.riscv.org/g/sw-dev/c/b55LI9OSFwo/m/d8-_NGmlAQAJ

I see that newlib/libc/include/sys/cdefs.h has

/*
 * 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

The flaw here is that C++ compilers don't define __STDC_VERSION so
this always causes __restrict to be defined away, even if the C++
compiler supports it.  This prevents use of the GNU extension
__restrict in C++ code.  The workaround is to use __restrict__ in user
code which GNU C++ also supports, and which newlib doesn't define
away.

I don't have a patch.  I'm hoping someone else writes one.  One
possible solution is to use a different keyword inside newlib, e.g.
__newlib_retrict.  This way we don't conflict with the compiler
__restrict feature.  Though this means newlib functions will be
declared without __restrict.  Another solution would be to check for
C++ compilers with __restrict support and support them.  Or maybe both
of these are appropriate.

We may not need the gcc-2.95 support anymore.

Jim