Move aligned memory utilities to rtc_base/memory/

This moves them from an API directory (system_wrappers/include/) to a
non-API directory, which is exactly what we want for utilities like
this.

BUG=webrtc:8445

Change-Id: I6dc34fe662f5d87b3b5288d33055345bc6bf91db
Reviewed-on: https://webrtc-review.googlesource.com/21164
Commit-Queue: Karl Wiberg <kwiberg@webrtc.org>
Reviewed-by: Danil Chapovalov <danilchap@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#22567}

rtc_base/BUILD.gn

@@ -1228,6 +1228,7 @@ if (rtc_include_tests) {
       "../system_wrappers:system_wrappers",
       "../test:fileutils",
       "../test:test_support",
+      "memory:unittests",
     ]
   }