Skip TestPluginCommands.py if host lldb library is incompatible with remote.

TestPluginCommands.py attempts to build a library which links against the host
built lldb library. This will only work if the remote is compatible with
binaries produced by the host.

Test Plan:
./dotest.py $DOTEST_OPTS -v -t -p TestPluginCommands.py
Test is skipped if remote platform is incompatible with host platform (i.e. mac
-> linux).

Differential Revision: http://reviews.llvm.org/D9770

llvm-svn: 237444