[LLDB][MIPS] Fix TestPrintStackTraces.py

Patch by Nitesh Jain.

Summary: The thread_start function in libc doesn't contain any epilogue and prologue instructions. Hence unwinding fail when we are stopped in thread_start.

Reviewers: ovyalov, clayborg
Subscribers: lldb-commits, mohit.bhakkad, sagar, bhushan, jaydeep
Differential: reviews.llvm.org/D16136
llvm-svn: 258685