[libc] Cache ownership of the shared buffer in the port

This patch adds another variable to cache cases where we know that we
own the buffer. This allows us to skip the atomic load on the inbox
because we already know its state. This is legal immediately after
opening a port, or when sending immediately after a recieve. This
caching nets a significant (~17%) speedup for the basic open, send,
recieve combination.

Reviewed By: JonChesterfield

Differential Revision: https://reviews.llvm.org/D150516