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/Value.cpp

@@ -934,6 +934,9 @@ ThrowCompletionOr<i32> Value::to_i32_slow_case(VM& vm) const
     // 1. Let number be ? ToNumber(argument).
     double number = TRY(to_number(vm)).as_double();
 
+#if __has_builtin(__builtin_arm_jcvt)
+    return __builtin_arm_jcvt(number);
+#else
     // 2. If number is not finite or number is either +0𝔽 or -0𝔽, return +0𝔽.
     if (!isfinite(number) || number == 0)
         return 0;
@@ -951,6 +954,7 @@ ThrowCompletionOr<i32> Value::to_i32_slow_case(VM& vm) const
     if (int32bit >= 2147483648.0)
         int32bit -= 4294967296.0;
     return static_cast<i32>(int32bit);
+#endif
 }
 
 // 7.1.8 ToInt16 ( argument ), https://tc39.es/ecma262/#sec-toint16

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);
 }