[RISCV] Fix logic to check if frame pointer should be used

Summary: The logic was broken for Linux triples as it returns true in the switch for Triple.isOSLinux().

Reviewers: asb, apazos

Reviewed By: asb

Subscribers: kito-cheng, shiva0217, cfe-commits

Differential Revision: https://reviews.llvm.org/D45237

llvm-svn: 329941