help complier to use intrinsics, clang in few cases e.g. aarch64 can not
and then requires linking with libm, its the only function needed from libm then
its good to avoid needing it.

Upstream-Status: Pending
Signed-off-by: Khem Raj <raj.khem@gmail.com>

--- a/include/fluent-bit/stream_processor/flb_sp_timeseries.h
+++ b/include/fluent-bit/stream_processor/flb_sp_timeseries.h
@@ -207,7 +207,7 @@ void cb_forecast_calc(struct timeseries
         result = b0 + b1 * (val->f64 + *forecast->latest_x);
         break;
     default:
-        result = nan("");
+        result = __builtin_nan("");
         break;
     }
 
@@ -283,7 +283,7 @@ void cb_forecast_r_calc(struct timeserie
             result = ((val->i64 - b0) / b1) - *forecast->latest_x;
             break;
         default:
-            result = nan("");
+            result = __builtin_nan("");
             break;
         }