Fixes to V8 GN build process on AIX

Change to v8/BUILD.gn regarding GN build of V8. Excluding platform-posix-time.h
and platform-posix-time.cc from the sources list on AIX os. The changes affect
AIX only.

Bug: chromium:616029
Change-Id: I5e5d46aa4c62f70544ff593294e459c607a03c67
Reviewed-on: https://chromium-review.googlesource.com/897124
Commit-Queue: Michael Achenbach <machenbach@chromium.org>
Reviewed-by: Michael Achenbach <machenbach@chromium.org>
Cr-Commit-Position: refs/heads/master@{#51183}

BUILD.gn

@@ -2589,11 +2589,15 @@ v8_component("v8_libbase") {
 
   if (is_posix) {
     sources += [
-      "src/base/platform/platform-posix-time.cc",
-      "src/base/platform/platform-posix-time.h",
       "src/base/platform/platform-posix.cc",
       "src/base/platform/platform-posix.h",
     ]
+    if (current_os != "aix") {
+      sources += [
+        "src/base/platform/platform-posix-time.cc",
+        "src/base/platform/platform-posix-time.h"
+      ]
+    }
   }
 
   if (is_linux) {