Set `TARGET_USES_64_BIT_BINDER` for mainline_sdk.

Devices are all using 64 bit binder, and mainline_sdk should too.
According the comment here: https://cs.android.com/android/platform/superproject/+/master:build/make/target/board/BoardConfigMainlineCommon.mk;l=25;drc=6e7693f1597a0934833a72faf35e918dfbdb6ecd,
this flag is required even for non-64 bit devices.

`libbinder` doesn't compile on host with 32 bit binder.

Test: packages/modules/common/build/mainline_modules_sdks.sh \
        --build-platform-sdks-for-mainline --build-release=latest
Change-Id: I357ce42e88ad13ed259d573b9dd2bbfc27b82687
diff --git a/target/board/mainline_sdk/BoardConfig.mk b/target/board/mainline_sdk/BoardConfig.mk
index 84f8b2d..f5c2dc6 100644
--- a/target/board/mainline_sdk/BoardConfig.mk
+++ b/target/board/mainline_sdk/BoardConfig.mk
@@ -18,3 +18,6 @@
 HOST_CROSS_OS := linux_bionic
 HOST_CROSS_ARCH := x86_64
 HOST_CROSS_2ND_ARCH :=
+
+# Required flag for non-64 bit devices from P.
+TARGET_USES_64_BIT_BINDER := true