[Sanitizer] Don't modify mmap_limit_mb flag in runtime.

Instead, rely on the fact that RAW_CHECK doesn't call mmap(),
and we'll be able to print an error message and kill a program.

git-svn-id: https://llvm.org/svn/llvm-project/compiler-rt/trunk@224034 91177308-0d34-0410-b5e6-96231b3b80d8