Add __va_copy and make it always visible, in spite of the __ISO_C_VISIBLE

setting.  Make va_copy be an alias if __ISO_C_VISIBLE >= 1999.

Why?  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 if
at all possible on amd64.  It generally isn't a problem for i386 and its
ilk because autoconf driven code tends to fall back to an assignment.

sys/amd64/include/stdarg.h

@@ -46,9 +46,12 @@ typedef	__va_list	va_list;
 #define	va_arg(ap, type) \
 	__builtin_va_arg((ap), type)
 
+#define	__va_copy(dest, src) \
+	__builtin_va_copy((dest), (src))
+
 #if __ISO_C_VISIBLE >= 1999
 #define	va_copy(dest, src) \
-	__builtin_va_copy((dest), (src))
+	__va_copy(dest, src)
 #endif
 
 #define	va_end(ap) \