[asan] Bump coverage size limit on 32-bit platforms.

This increases the limit from 4M locations to 16M, reserving
64Mb virtual memory. Chrome has >5M unique coverage locations with coverage=3.


git-svn-id: https://llvm.org/svn/llvm-project/compiler-rt/trunk@224855 91177308-0d34-0410-b5e6-96231b3b80d8