Add lowercase OS name feature

Summary:
Some tests already make use of OS feature names, e.g. 'linux' and 'freebsd',
but they are not actually currently set by lit.

Reviewers: pcc, eugenis

Reviewed By: eugenis

Subscribers: emaste, krytarowski, delcypher, llvm-commits, #sanitizers

Differential Revision: https://reviews.llvm.org/D49115

llvm-svn: 336633