Disable gmodules tests on linux

These tests are extremely environment-dependent. if the environment is
not module-enabled (which is the likely scenario), they won't test
anything. If one happens to have a module-enabled libc++, then the he
will start running into problems.

The first one is that the debug info in pcm file contains relocations
that ObjectFileELF doesn't handle (particularly on non-x86
architectures), but even after that is resolved, it seems we still are
unable to pull debug info out of the pcm file. I've filed pr37893 to
track that, and I am disabling gmodules tests on linux until these
issues are resolved.

llvm-svn: 335235