Made GetClangTargetCPU() const.

Summary:
It does not change members or call non-const members.
HostInfo::GetArchitecture() returns a const object ref (maybe others?),
which can't access the non-const function.

Reviewers: labath, eugene

Reviewed By: labath, eugene

Subscribers: lldb-commits

Differential Revision: https://reviews.llvm.org/D30515
Author:	Jason Majors <jmajors@google.com>

llvm-svn: 296868