[compiler-rt][asan|win] Fix flaky unittest due to large allocations
Coverage is using large arrays which requires large allocations.
These allocations are flaky and often failing on win64.
We are using the 32-bits size until this gets a better fix.
Reviewed By: rnk
Subscribers: llvm-commits, kubamracek, chrisha, dberris
Differential Revision: https://reviews.llvm.org/D29945