thrift: add temporary Mutex implementation

The Thrift library has its own abstraction for mutexes, which
normally just a wrapper around `std::mutex` using the PIMPL
idiom (pointer-to-impl).

Since Zephyr does not yet support `std::mutex`, a workaround
was added in Zephyr that was essentially no-op, and actually
the PIMPL idiom made it quite easy to do that. However,
pretending there is no synchronization requirement is not a
solution for it.

We can't yet just use a `struct k_mutex` yet, because we
don't yet support userspace, but for now we can fake a mutex
interface with a spinlock.

We hope to eventually drop this workaround entirely and just
support `std::mutex`.

Signed-off-by: Christopher Friedt <cfriedt@meta.com>