Make fallback fegetexceptflag work with generic fetestexceptflag.

The generic implementation of fetestexceptflag does:

int
fetestexceptflag (const fexcept_t *flagp, int excepts)
{
  /* Most versions of fegetexceptflag store exceptions in a form such
     that this works.  */
  return *flagp & excepts & FE_ALL_EXCEPT;
}

In the case where FE_ALL_EXCEPT is nonzero but exceptions may not be
supported at runtime, this only works if fegetexceptflag cleared all
the bits of FE_ALL_EXCEPT in *flagp; otherwise it accesses
uninitialized data.  This showed up as a failure of
math/test-fetestexceptflag for MIPS o32 soft-float.  This patch makes
the fallback fegetexceptflag store 0 (fexcept_t is an integer type
everywhere) so that this works.  (No bug report in Bugzilla because
this wasn't user-visible - at least, without using tools to detect
uninitialized memory use at runtime - without fetestexceptflag, which
is new in 2.25.)

Tested for MIPS o32 soft-float.

	* math/fgetexcptflg.c (__fegetexceptflag): Store 0 in fexcept_t
	object.

ChangeLog

@@ -1,3 +1,8 @@
+2017-01-12  Joseph Myers  <joseph@codesourcery.com>
+
+	* math/fgetexcptflg.c (__fegetexceptflag): Store 0 in fexcept_t
+	object.
+
 2017-01-11  Joseph Myers  <joseph@codesourcery.com>
 
 	[BZ #16458]

math/fgetexcptflg.c

@@ -24,6 +24,7 @@ int
 __fegetexceptflag (fexcept_t *flagp, int excepts)
 {
   /* Nothing to do.  */
+  *flagp = 0;
   return 0;
 }
 #if SHLIB_COMPAT (libm, GLIBC_2_1, GLIBC_2_2)