Add LIT_USE_INTERNAL_SHELL to compiler-rt tests.

I am working on improving our internal bot infrastructure. One thing
that is unique to the ps4 is that we want to run the posix tests, but
have to execute them on windows.

We currently have a local hack to use a shell on windows, but it is
pretty much impossible to get all all the tools to play nice with all
the heuristics for what is a path and what is a command line option.

This adds support LIT_USE_INTERNAL_SHELL and I will then try to fix
the tests that fail with it but adding the missing features.

llvm-svn: 299077