Fix boundary case in log2_extended_precision_half_safe.

log2_extended_precision_half_safe is a modified version of
log2_extended_precision_half_unsafe that avoids denormal intermediate
results via scaling. The unsafe version gives one input, 0x39f6, special
treatment because the approximation is known to be incorrect, but the
safe version did not translate this special treatment correctly: the
special casing happens before downscaling, therefore the constant needs
to be upscaled.

This fixes a pow test failure on FTZ targets.

modules/compiler/builtins/abacus/include/abacus/internal/log2_extended_precision.h

@@ -280,7 +280,8 @@ T log2_extended_precision_half_safe(const T &x, T *ans_lo, T *hiExp, T *loExp) {
 
   // Single awkward boundary value fix:
   const SignedType edge(abacus::detail::cast::as<UnsignedType>(x) == 0x39f6);
-  remainder = __abacus_select(remainder, T(-0.000144362f16), edge);
+  // -0.14783 ==> -0.000144362 * 2^10
+  remainder = __abacus_select(remainder, T(-0.14783f16), edge);
 
   // Set return parameters
   *hiExp = abacus::detail::cast::convert<T>(hiExpI);