[LSan] Enable LSan tests on PPC64 Linux.

Summary:
LSan is functional on PPC64 Linux now, let's enable all tests.

One test required ppc specific changes: use_registers.cc.

Reviewers: eugenis

Subscribers: mgorny, llvm-commits

Differential Revision: https://reviews.llvm.org/D39316

llvm-svn: 316698