Add a new target.process.memory-cache-line-size to change the size of

lldb's internal memory cache chunks that are read from the remote
system.  For a remote connection that is especially slow, a user may
need to reduce it; reading a 512 byte chunk of memory whenever a
4-byte region is requested may not be the right decision in these
kinds of environments.
<rdar://problem/18175117> 

llvm-svn: 217083