[PowerPC][asan] Fix asan tests to handle changed memory layouts

In more recent Linux kernels with 47 bit VMAs the layout of virtual memory
for powerpc64 changed causing the address sanitizer to not work properly. This
patch fixes up a test case that was found to fail on some newer Fedora
releases that use different address ranges.

ref: https://reviews.llvm.org/D40907
llvm-svn: 323217