For IBM long double, adding a normal number to a NaN raises an inexact exception. Adding any number to NaN should not raise any exception. The following testcase triggers the issue (the testcase is meant to run a gnu compatible libc):

--------------------------------------------------------------
$ cat gcc_testcase.c
#include <math.h>
#include <fenv.h>
#include <stdio.h>

double
sum (double x, long double y)
{
  return x + y;
}

int main ()
{
  feenableexcept (FE_INEXACT);

  double x = __builtin_nan ("");
  long double y = 1.1L;

  printf ("%e\n", sum (x, y));
  
  return 0;
}
$ gcc -O3 -m64 -fno-inline gcc_testcase.c -o gcc_testcase -lm
$ ./gcc_testcase 
Floating point exception (core dumped)
--------------------------------------------------------------

The issue is in __gcc_qadd implementation at libgcc/config/rs6000/ibm-ldouble.c, 
if the number if non finite, there is not check if it a NaN before actually summing all the components. A possible solution would be to add an extra test and return the first sum if the number if not infinity:

Index: libgcc/config/rs6000/ibm-ldouble.c
===================================================================
--- libgcc/config/rs6000/ibm-ldouble.c  (revision 199159)
+++ libgcc/config/rs6000/ibm-ldouble.c  (working copy)
@@ -104,6 +104,8 @@
 
   if (nonfinite (z))
     {
+      if (z != inf())
+       return z;
       z = cc + aa + c + a;
       if (nonfinite (z))
        return z;