Add linux_bionic cross compilation for mainline SDKs. am: 7356fffd

mkdir -p ${SOONG_OUT}

SOONG_VARS=${SOONG_OUT}/soong.variables

# We enable bionic linux builds as ART also needs prebuilts for it.

# Enabling bionic linux requires setting allow_missing_dependencies.

cat > ${SOONG_VARS}.new << EOF

{

    "Platform_sdk_version": ${PLATFORM_SDK_VERSION},

    "Platform_sdk_codename": "${PLATFORM_VERSION}",

    "Platform_version_active_codenames": ${PLATFORM_VERSION_ALL_CODENAMES},

    "Allow_missing_dependencies": true,

    "DeviceName": "generic_arm64",

    "HostArch": "x86_64",

    "HostSecondaryArch": "x86",

    "CrossHost": "linux_bionic",

    "CrossHostArch": "x86_64",

    "Aml_abis": true,

    "UseGoma": ${USE_GOMA}