Simplify GetGlobalProperties functions of Thread/Process/Target

Summary:
"Initialization of function-local statics is guaranteed to occur only once even when called from
multiple threads, and may be more efficient than the equivalent code using std::call_once."
<http://en.cppreference.com/w/cpp/thread/call_once>

I'd add that it's also more readable.

Reviewers: clayborg, zturner

Subscribers: lldb-commits

Differential Revision: http://reviews.llvm.org/D17710

llvm-svn: 284601