[libc][rpc] Simplify mailbox state tracking

Removes the redundant Ack/Data bit manipulation.

Represents the inbox/outbox state with one bit instead of two. This will
be useful if we change to a packed representation and otherwise cuts the
runtime state space from 16 to 4.

Further simplification is possible, this patch is intentionally minimal.
- can_{send,recv}_data are now in == out
- {client,server}::try_open can be factored into Process:try_open

This implements the state machine of D148191, modulo differences in atomic
ordering and fences.

Reviewed By: jhuber6

Differential Revision: https://reviews.llvm.org/D149788