ART: Add a test checking TLS offsets and entrypoints

Add a test (runtime for now, as compile-time checks seem not
possible with current infrastructure) that checks the structure
of the thread tls and entrypoint structs. A failure of the test
indicates a structural change, which hints at a necessary oat
version bump.

Change-Id: I500b5cd940c916ebfc1580e9dd2c287c4d263956