build x86_64 binary to accomodate google play (#1430)

.travis.yml

@@ -14,9 +14,10 @@ android:
 # Install Android NDK (apparently there is no easier way to do this)
 # https://github.com/travis-ci/travis-ci/issues/5395
 before_script:
-  - curl -L https://dl.google.com/android/repository/android-ndk-r15c-linux-x86_64.zip -O
-  - unzip -q android-ndk-r15c-linux-x86_64.zip
-  - export ANDROID_NDK_HOME=`pwd`/android-ndk-r15c
+  - export NDK_VERSION=r20
+  - curl -L https://dl.google.com/android/repository/android-ndk-${NDK_VERSION}-linux-x86_64.zip -O
+  - unzip -q android-ndk-${NDK_VERSION}-linux-x86_64.zip
+  - export ANDROID_NDK_HOME=`pwd`/android-ndk-${NDK_VERSION}
 
 before_install:
   # Install Go using gimme tool

docker/prebuild.sh

@@ -2,7 +2,7 @@
 
 [ -z "$SYNCTHING_ANDROID_PREBUILT" ] && echo "Prebuild disabled" && exit 0
 
-for ARCH in arm x86 arm64; do
+for ARCH in arm x86 arm64 x86_64; do
   GOARCH=${ARCH}
   SDK=16
   case ${ARCH} in
@@ -17,6 +17,10 @@ for ARCH in arm x86 arm64; do
         GOARCH=386
         GCC="i686-linux-android-clang"
         ;;
+      x86_64)
+        SDK=21
+        GCC="x86_64-linux-android21-clang"
+        ;;
       *)
         echo "Invalid architecture"
         exit 1

syncthing/build-syncthing.py

@@ -26,6 +26,13 @@ BUILD_TARGETS = [
         'goarch': '386',
         'jni_dir': 'x86',
         'cc': 'i686-linux-android-clang',
+    },
+    {
+        'arch': 'x86_64',
+        'goarch': 'amd64',
+        'jni_dir': 'x86_64',
+        'cc': 'x86_64-linux-android21-clang',
+        'min_sdk': 21,
     }
 ]