cc: clearer OS mismatch error.

Bugs: me
Test: clear error message, for instance:

    error: frameworks/native/libs/binder/ndk/Android.bp:252:1: module "libbinder_ndk.ndk" variant "android_x86_64_silvermont_sdk_shared_29": OS mismatch between "libbinder_ndk.ndk" (android) and "libbinder_headers_platform_shared" (linux_glibc)

Change-Id: Idfa6d27daeb89056781c93a893e4080e49b3fcb0