Fix 64-bit fortran options for riscv64

64-bit builds are currently broken without this flag.

Makefiles have done this already: 5720fa02c5/Makefile.system (L831)

cmake/fc.cmake

@@ -63,6 +63,11 @@ if (${F_COMPILER} STREQUAL "GFORTRAN" OR ${F_COMPILER} STREQUAL "F95")
         set(FCOMMON_OPT "${FCOMMON_OPT} -mabi=lp32")
       endif ()
     endif ()
+    if (RISCV64)
+      if (BINARY64)
+        set(FCOMMON_OPT "${FCOMMON_OPT} -fdefault-integer-8")
+      endif ()
+    endif ()
   else ()
     if (BINARY64)
       set(FCOMMON_OPT "${FCOMMON_OPT} -m64")