[builtins] stop using imprecise fdlibm pow

This CL reinstates the old pow implementation which calls out to the
system implementation of pow.

Bug: v8:9622
Change-Id: I3df997888ced3fb8b5bd4b810098e967649aaa55
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/1774898Reviewed-by: Hannes Payer <hpayer@chromium.org>
Reviewed-by: Georg Neis <neis@chromium.org>
Commit-Queue: Georg Neis <neis@chromium.org>
Cr-Commit-Position: refs/heads/master@{#66303}

test/mjsunit/regress/regress-9622.js

+// Copyright 2019 the V8 project authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+'use strict';
+
+assertEquals(0.6840442338072671 ** 2.4, 0.4019777798321958);
+
+const constants = {
+  '0': 1,
+  '-1': 0.1,
+  '-2': 0.01,
+  '-3': 0.001,
+  '-4': 0.0001,
+  '-5': 0.00001,
+  '-6': 0.000001,
+  '-7': 0.0000001,
+  '-8': 0.00000001,
+  '-9': 0.000000001,
+};
+
+for (let i = 0; i > -10; i -= 1) {
+  assertEquals(10 ** i, constants[i]);
+  assertEquals(10 ** i, 1 / (10 ** -i));
+}