tsan: add debugging code for ptrace test failures

Debugging of crashes on powerpc after commit:
c80604f7 ("tsan: remove real func check from interceptors")
Somehow replacing if with DCHECK leads to strange failures in:
SanitizerCommon-tsan-powerpc64le-Linux :: Linux/ptrace.cpp
https://lab.llvm.org/buildbot/#/builders/105
https://lab.llvm.org/buildbot/#/builders/121
https://lab.llvm.org/buildbot/#/builders/57

The hypothesis is that something writes out-of-bounds
into pt_regs on stack and that corrupts internal tsan state.

Reviewed By: melver

Differential Revision: https://reviews.llvm.org/D112703