[compiler-rt][asan|win] Fix flaky unittest due to large allocations

Summary:
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.

Reviewers: rnk

Reviewed By: rnk

Subscribers: llvm-commits, kubamracek, chrisha, dberris

Differential Revision: https://reviews.llvm.org/D29945

llvm-svn: 295086