Change the amount of data that Platform::PutFile will try to transfer

in one packet from 1k bytes to 16k bytes.  Sending a large file to an
iOS device directly connected by USB cable, to lldb-server running in
platform mode, this speeds up the file xfer by 77%.  Sending the file
in 32k blocks speeds up the file xfer by 80% versus 1k blocks, starting
with 16k to make sure we don't have any problems with android testing.

We may not have the same perf characteristics over ethernet, but with
USB it's faster to send fewer larger packets than many small packets.

llvm-svn: 348557