Add system-linux to allow tests run with llvm-lit to restrict themselves to linux

I need a test that only runs in a reasonable amount of time on systems
that have sparse files. The broadest class of systems that support
sparse files are linux systems. So restricting my test to linux systems
should suffice. This change adds the system-linux feature to llvm-lit so
that it can be required.

Differential Revision: https://reviews.llvm.org/D39482

llvm-svn: 317055