fpu: Bound increment for scalbn

Without bounding the increment, we can overflow exp either here
in scalbn_decomposed or when adding the bias in round_canonical.
This can result in e.g. underflowing to 0 instead of overflowing
to infinity.

The old softfloat code did bound the increment.

Backports commit ce8d4082054519f2eaac39958edde502860a7fc6 from qemu

qemu/fpu/softfloat.c

@@ -1878,6 +1878,12 @@ static FloatParts scalbn_decomposed(FloatParts a, int n, float_status *s)
         return return_nan(a, s);
     }
     if (a.cls == float_class_normal) {
+        /* The largest float type (even though not supported by FloatParts)
+         * is float128, which has a 15 bit exponent.  Bounding N to 16 bits
+         * still allows rounding to infinity, without allowing overflow
+         * within the int32_t that backs FloatParts.exp.
+         */
+        n = MIN(MAX(n, -0x10000), 0x10000);
         a.exp += n;
     }
     return a;