Change ProcessGDBRemote::DoReadMemory to use the x packet to read
data if it is available.

Change ProcessGDBRemote's maximum read/write packet size from a
fixed 512 byte value to asking the remote gdb stub what its maximum
is, using up to 128kbyte sizes if that's allowed, and falling back
to 512 if the remote gdb stub doesn't advertise a max packet size.

Add a new "process plugin packet xfer-size" command that can be used
to override the maximum packet size (although not exceeding any packet
size maximum published by the remote gdb stub).
<rdar://problem/16032150>