[asan] Scale back mmap_limit_mb test.

There is some strange interaction between mmap limit and unlimited stack
(ulimit -s unlimited), which results in this test failing when run with
"make".


git-svn-id: https://llvm.org/svn/llvm-project/compiler-rt/trunk@218764 91177308-0d34-0410-b5e6-96231b3b80d8