Add support for 64-bit NDKs to Android Plugin.

Currently the Android plugin only checks for the linux-x86 prebuilds in the
64-bit Android NDK (and mingw-and-ndk), this means Qt Creator goes bonkers
because it can't find the toolchain and debugger for the kits it
auto-detects/creates.

A work around is to symlink linux-x86_64 to linux-x86 in every toolchain.

Change-Id: I04522b65ef48b6090a9f6925e8e3420ad1d333ee
Reviewed-by: Paul Olav Tvete <paul.tvete@digia.com>
Reviewed-by: Daniel Teske <daniel.teske@digia.com>

src/plugins/android/androidsettingswidget.cpp

@@ -286,6 +286,7 @@ void AndroidSettingsWidget::sdkLocationEditingFinished()
 void AndroidSettingsWidget::ndkLocationEditingFinished()
 {
     Utils::FileName location = Utils::FileName::fromUserInput(m_ui->NDKLocationLineEdit->text());
+    m_androidConfig.toolchainHost.clear(); // force toolchain host detection
     if (!checkNDK(location))
         return;
     saveSettings(true);