Move Host::GetArchitecture to HostInfo::GetArchitecture.

As a side effect, this patch also eliminates all of the
preprocessor conditionals previously used to implement
GetArchitecture().

git-svn-id: https://llvm.org/svn/llvm-project/lldb/trunk@216074 91177308-0d34-0410-b5e6-96231b3b80d8