Add cross-platform NDK repository support

[UebelAndre]

This change enables workflows where the host Bazel platform differs from the exec platforms via remote execution. When creating a local_repository to an NDK for another platform, the repository rules are now flexible and platform-agnostic enough to support correctly matching foreign exec platforms if explicitly specified.

android_ndk_repository(
    name = "android_ndk_linux",
    anchor = "@local_android_ndk_linux//:BUILD.bazel",
    api_level = 21,
    platform = "linux-x86_64",
    clang_resource_dir = "linux/specific/dir",
)