[lit] Add pthread to the compilation of the tests on Linux

Summary: Right now only some platforms add pthread to the compilation, however, at least one of the tests requires the pthread library on Linux as well. Since the library is available, this change adds it by default on Linux.

Reviewers: labath, zturner, asmith

Subscribers: stella.stamenova, jfb, lldb-commits

Differential Revision: https://reviews.llvm.org/D54808

llvm-svn: 347412