Turn symbolization on for ASan unit test.

Summary:
On PowerPC and ARM (possibly, need to verify), couple tests involving
pthread_exit fail due to leaks detected by LSan. pthread_exit tries
to perform unwinding that leads to dlopen'ing libgcc_s.so. dlopen
mallocs "libgcc_s.so" string which confuses LSan, it fails to
realize that this allocation happens in dynamic linker and should
be ignored.
Symbolized leak report is required to define a suppression for this
known problem.

Reviewers: eugenis

Subscribers: aemerson, rengolin, kubamracek, llvm-commits

Differential Revision: https://reviews.llvm.org/D32194

Turn symbolization on for PPC and Thumb only to do not slow down other platforms.

llvm-svn: 300748