ArchSpec: Do not depend on Host::GetArchitecture.

The major issue this patch solves is that ArchSpec::SetTriple no longer depends
on the implementation of Host::GetArchitecture.  On linux, Host::GetArchitecture
calls ArchSpec::SetTriple, thus blowing the stack.

A second smaller point is that SetTriple now defaults to Host defined components
iff all OS, vendor and environment fields are not set.

llvm-svn: 126403