[ASan] Replace a CHECK for mmap_limit_mb with a RAW_CHECK.
In the case of a CHECK failure the program tries to fork and launch llvm-symbolizer,
but hangs in mz_force_lock because one of the allocator locks is already acquired.


git-svn-id: https://llvm.org/svn/llvm-project/compiler-rt/trunk@206274 91177308-0d34-0410-b5e6-96231b3b80d8