In the spirit of amd64/include/stdarg.h rev 1.6; add __va_copy

(but keep it conditional on __ISO_C_VISIBLE >= 1999.

Why?  Our out /usr/src/contrib assumes it, and more than a few ports have
an autoconf that looks for __va_copy because it is available on glibc.
It is critical that we use it on PowerPC.  It generally isn't a problem
for i386 and its ilk because those platforms can get away with cheating
the C standard, using a plain assignment.

sys/powerpc/include/stdarg.h

@@ -50,6 +50,9 @@ typedef	__va_list	va_list;
 #if __ISO_C_VISIBLE >= 1999
 #define	va_copy(dest, src) \
 	__builtin_va_copy((dest), (src))
+
+#define	__va_copy(dest, src) \
+	va_copy((dest), (src))
 #endif
 
 #define	va_end(ap) \