fenv: add missing declarations to x86 fenv.h

feenableexcept, fedisableexcept and fegetexcept were
accidentally missing in the x86 fenv.h

Signed-off-by: Corinna Vinschen <corinna@vinschen.de>

newlib/libc/machine/x86_64/sys/fenv.h

@@ -156,6 +156,11 @@ extern const fenv_t *_fe_dfl_env;
    if _GNU_SOURCE is defined.  */
 extern const fenv_t *_fe_nomask_env;
 #define FE_NOMASK_ENV (_fe_nomask_env)
+
+/* These are GNU extensions defined in glibc.  */
+int feenableexcept (int __excepts);
+int fedisableexcept (int __excepts);
+int fegetexcept (void);
 #endif /* __GNU_VISIBLE */
 
 #ifdef __INSIDE_CYGWIN__