[llvm-exegesis] Adjust page size in unit tests to fix ppc failures

The llvm-exegesis unit tests currently fail on PPC after
ceb196d9 landed as the default page size
on most common linux distributions for PPC is 64kb rather than 4kb. This
patch changes the memory mappings to have addresses as multiples of 64kb
rather than multiples of 4kb to fix this issue.