Add GN build support for Android 64-bit platforms.

build/config/android/config.gni

Index: build/config/android/config.gni
diff --git a/build/config/android/config.gni b/build/config/android/config.gni
index 60934c2e0b54a5300681e87663e1b16b43d00ca6..6cb3a6ad4386352f2416bf2ee9d7add6f05aac8d 100644
--- a/build/config/android/config.gni
+++ b/build/config/android/config.gni
@@ -93,6 +93,13 @@ if (is_android) {
       "platforms/android-${_android_api_level}/arch-arm"
   mips_android_sysroot_subdir =
       "platforms/android-${_android_api_level}/arch-mips"
+  _android64_api_level = 21
+  x86_64_android_sysroot_subdir =
+      "platforms/android-${_android64_api_level}/arch-x86_64"
+  arm64_android_sysroot_subdir =
+      "platforms/android-${_android64_api_level}/arch-arm64"
+  mips64_android_sysroot_subdir =
+      "platforms/android-${_android64_api_level}/arch-mips64"
 
   # Toolchain root directory for each build. The actual binaries are inside
   # a "bin" directory inside of these.
@@ -100,6 +107,9 @@ if (is_android) {
   x86_android_toolchain_root = "$android_ndk_root/toolchains/x86-${_android_toolchain_version}/prebuilt/${android_host_os}-${android_host_arch}"
   arm_android_toolchain_root = "$android_ndk_root/toolchains/arm-linux-androideabi-${_android_toolchain_version}/prebuilt/${android_host_os}-${android_host_arch}"
   mips_android_toolchain_root = "$android_ndk_root/toolchains/mipsel-linux-android-${_android_toolchain_version}/prebuilt/${android_host_os}-${android_host_arch}"
+  x86_64_android_toolchain_root = "$android_ndk_root/toolchains/x86_64-${_android_toolchain_version}/prebuilt/${android_host_os}-${android_host_arch}"
+  arm64_android_toolchain_root = "$android_ndk_root/toolchains/aarch64-${_android_toolchain_version}/prebuilt/${android_host_os}-${android_host_arch}"
+  mips64_android_toolchain_root = "$android_ndk_root/toolchains/mips64el-${_android_toolchain_version}/prebuilt/${android_host_os}-${android_host_arch}"
 
   # Location of libgcc. This is only needed for the current GN toolchain, so we
   # only need to define the current one, rather than one for every platform
@@ -119,6 +129,21 @@ if (is_android) {
     _binary_prefix = "mipsel-linux-android"
     android_toolchain_root = "$mips_android_toolchain_root"
     android_libgcc_file = "$android_toolchain_root/lib/gcc/mipsel-linux-android/${_android_toolchain_version}/libgcc.a"
+  } else if (cpu_arch == "x64") {
+    android_prebuilt_arch = "android-x86_64"
+    _binary_prefix = "x86_64-linux-android"
+    android_toolchain_root = "$x86_64_android_toolchain_root"
+    android_libgcc_file = "$android_toolchain_root/lib/gcc/x86_64-linux-android/${_android_toolchain_version}/libgcc.a"
+  } else if (cpu_arch == "arm64") {
+    android_prebuilt_arch = "android-arm64"
+    _binary_prefix = "aarch64-linux-android"
+    android_toolchain_root = "$arm64_android_toolchain_root"
+    android_libgcc_file = "$android_toolchain_root/lib/gcc/aarch64-linux-android/${_android_toolchain_version}/libgcc.a"
+  } else if (cpu_arch == "mips64el") {
+    android_prebuilt_arch = "android-mips64"
+    _binary_prefix = "mips64el-linux-android"
+    android_toolchain_root = "$mips64_android_toolchain_root"
+    android_libgcc_file = "$android_toolchain_root/lib/gcc/mips64el-linux-android/${_android_toolchain_version}/libgcc.a"
   } else {
     assert(false, "Need android libgcc support for your target arch.")
   }
@@ -150,6 +175,12 @@ if (is_android) {
     }
   } else if (cpu_arch == "mipsel") {
     android_app_abi = "mips"
+  } else if (cpu_arch == "x64") {
+    android_app_abi = "x86_64"
+  } else if (cpu_arch == "arm64") {
+    android_app_abi = "arm64-v8a"
+  } else if (cpu_arch == "mips64el") {
+    android_app_abi = "mips64"
   } else {
     assert(false, "Unknown Android ABI: " + cpu_arch)
   }