LibJS: Make use of arm64 FJCVTZS instruction if available

FJCVTZS (Floating-point Javascript Convert to Signed fixed-point,
rounding toward Zero) does exactly what we need for ToInt32 in the
JavaScript specification.

This isn't world-changing, but it does give a ~2% boost on compute-
heavy benchmarks like JetStream, so we should obviously use it.

Libraries/LibJS/Runtime/ValueInlines.h

@@ -54,6 +54,11 @@ inline ThrowCompletionOr<i32> Value::to_i32(VM& vm) const
     if (is_int32())
         return as_i32();
 
+#if __has_builtin(__builtin_arm_jcvt)
+    if (is_double())
+        return __builtin_arm_jcvt(m_value.as_double);
+#endif
+
     return to_i32_slow_case(vm);
 }