In ProcessGDBRemote::DoConnectRemote(), if the remote system informed

us of its architecture, use that to set the Target's arch if it
doesn't already have one set.

In Process::CompleteAttach(), if the Target has a valid arch make
sure that the Platform we pick up is compatible with that arch; if
not, find a Platform that is compatible.  Don't let the the default
platform override the Target's arch.

<rdar://problem/11185420>

llvm-svn: 156116