Don't include UnixSignals.h from Host.

Host had a function to get the UnixSignals instance corresponding
to the current host architecture.  This means that Host had to
include a file from Target.  To break this dependency, just make
this a static function directly in UnixSignals.  We already have
the function UnixSignals::Create(ArchSpec) anyway, so we just
need to have UnixSignals::CreateForHost() which determines which
value to pass for the ArchSpec.

The goal here is to eventually break the Host->Target->Host
circular dependency.

Differential Revision: https://reviews.llvm.org/D57780

llvm-svn: 354168