[PowerPC] Update MemorySanitizer test to cater for number of CPUs > 1024

MemorySanitizer test fails on systems with more than 1024 CPUs.
This patch updates the test to make it work for machines that
have more than 1024 CPUs. This helps to fix errors on the PowerPC
sanitizer bot.

Reviewed By: #powerpc, nemanjai

Differential Revision: https://reviews.llvm.org/D87053