[fpe] in gq_ttq.mad, fix the SIGFPE crash for FPTYPE=f 512z builds (m…

…adgraph5#845) by adding 'volatile', again

This no longer crashes

gdb ./madevent_cpp -ex 'set pagination off' -ex 'set confirm off' -ex 'set trace-commands on' \
  -ex 'run < /tmp/avalassi/input_gqttq_x1_cudacpp' -ex where -ex l -ex 'p okcol' -ex quit

epochX/cudacpp/gq_ttq.mad/SubProcesses/P1_gu_ttxu/CPPProcess.cc

@@ -1190,7 +1190,8 @@ namespace mg5amcCpu
           for( int icolC = 0; icolC < ncolor; icolC++ )
           {
 #if defined MGONGPU_CPPSIMD
-            const bool okcol = allrndcol[ievt] < ( targetamp[icolC][ieppV] / targetamp[ncolor - 1][ieppV] );
+            // Add volatile here to avoid SIGFPE crashes in FPTYPE=f cpp512z builds (#845)
+            volatile const bool okcol = allrndcol[ievt] < ( targetamp[icolC][ieppV] / targetamp[ncolor - 1][ieppV] );
 #else
             const bool okcol = allrndcol[ievt] < ( targetamp[icolC] / targetamp[ncolor - 1] );
 #endif