Select expected remote libc++ library name based on remote platform.

Select expected remote libc++ library name based on remote platform. This is
used to verify libc++ is in the loaded image list by various tests which use it.

Test Plan:
Passes the following tests mac -> linux with this patch:
TestDataFormatterLibccIterator.py
TestDataFormatterLibccMap.py
TestDataFormatterLibccMultiMap.py
TestDataFormatterLibcxxMultiSet.py
TestDataFormatterLibcxxSet.py
TestDataFormatterUnordered.py

Differential Revision: http://reviews.llvm.org/D9759

llvm-svn: 237288