[orc-rt] Add printf logging backend (#209006)
Implement the printf backend, selected at configure-time with
ORC_RT_LOG_BACKEND=printf. Messages are written to stderr, or to the
file named by the ORC_RT_LOG_OUTPUT environment variable.
The ORC_RT_LOG environment variable sets the runtime level floor (error,
warning, info, debug, or off) -- only messages at or above it are shown.
When unset it defaults to warning, so warnings and errors are shown and
info and debug are opt-in. The runtime floor cannot go below the
compile-time ORC_RT_LOG_LEVEL floor (the lowest level built in).
Also add regression tests, driven by a new orc-rt-log-check tool that
reports the compiled-in backend and levels (which lit exposes as
features) and emits a message at each level.
