[libc] More efficiently send bytes via `send_n` and `recv_n`

Currently we have the `send_n` and `recv_n` routines to stream data,
such as a string to print, to the other side. The first operation is to
send the size so the other side knows the number of bytes to recieve.
However, this wasted 56 bytes that could've been sent. This meant that
small values, like the arguments to a function to call on the host for
example, needed to perform an extra send. This patch sends the first 56
bytes in the first packet and continues if necessary.

Depends on D150992

Reviewed By: JonChesterfield

Differential Revision: https://reviews.llvm.org/D151041