nuttx/libm: Fix powf wrong if first parameter is negative

Signed-off-by: yanghuatao <yanghuatao@xiaomi.com>

libs/libm/libm/lib_powf.c

@@ -37,5 +37,21 @@
 
 float powf(float b, float e)
 {
-  return expf(e * logf(b));
+  if (b > 0.0)
+    {
+      return expf(e * logf(b));
+    }
+  else if (b < 0.0 && e == (int)e)
+    {
+      if ((int)e % 2 == 0)
+        {
+          return expf(e * logf(fabsf(b)));
+        }
+      else
+        {
+          return -expf(e * logf(fabsf(b)));
+        }
+    }
+
+  return 0.0;
 }