Uses the target platform to determine the platform context.

Uses the target platform to determine the platform context to use. This fixes an
early failure in some tests when running on a different remote target.

Test Plan:
./dotest.py $DOTEST_OPTS -t -p TestGlobalVariables.py|TestSharedLib.py
Running on macosx -> linux, this fails at a later stage in the test - rather
than in registerSharedLibrariesWithTarget looking for a library without the
correct "lib" prefix and ".so" suffix.

Differential Revision: http://reviews.llvm.org/D9047

llvm-svn: 235164