[esan] Add handling of large stack size rlimits

Summary:
Adds detection of large stack size rlimits (over 1 TB or unlimited), which
results in an mmap location that our shadow mapping does not support.  We
re-exec the application in this situation.  Adds a test of this behavior.

Adds general detection of mmap regions outside of our app regions.  In the
future we want to try to adaptively handle these but for now we abort.

Moves the existing Linux-specific mmap code into a platform-specific file
where the new rlimit code lives.

Reviewers: eugenis

Subscribers: vitalybuka, zhaoqin, kcc, aizatsky, llvm-commits, kubabrecka

Differential Revision: http://reviews.llvm.org/D20745

llvm-svn: 271079