Remove last Host usage from ArchSpec

Summary:
In D39387, I was quick to jump to conclusion that ArchSpec has no
external dependencies. It turns there still was one call to
HostInfo::GetArchitecture left -- for implementing the "systemArch32"
architecture and friends.

Since GetAugmentedArchSpec is the place we handle these "incomplete"
triples that don't specify os or vendor and "systemArch" looks very much
like an incomplete triple, I move its handling there.

After this ArchSpec *really* does not have external dependencies, and
I'll move it to the Utility module as a follow-up.

Reviewers: zturner, clayborg, jingham

Subscribers: lldb-commits

Differential Revision: https://reviews.llvm.org/D39896

llvm-svn: 318046