[libc] Change RPC outbox stores to be relaxed

Summary:
These stored previously used `RELEASE`. This was done originally to
ensure that the stores to the shared memory buffer were flushed prior to
signaling that the other side can begin accessing it. However, this
should be accomplished by the memory fence above the store. This change
is required because NVPTX does not support non-relaxed atomics used on
unified shared memory.