[sanitizer] [SystemZ] Abort if the kernel might be vulnerable to CVE-2016-2143.

In short, CVE-2016-2143 will crash the machine if a process uses both >4TB
virtual addresses and fork().  ASan, TSan, and MSan will, by necessity, map
a sizable chunk of virtual address space, which is much larger than 4TB.
Even worse, sanitizers will always use fork() for llvm-symbolizer when a bug
is detected.  Disable all three by aborting on process initialization if
the running kernel version is not known to contain a fix.

Unfortunately, there's no reliable way to detect the fix without crashing
the kernel.  So, we rely on whitelisting - I've included a list of upstream
kernel versions that will work.  In case someone uses a distribution kernel
or applied the fix themselves, an override switch is also included.

Differential Revision: http://reviews.llvm.org/D19576

llvm-svn: 267747

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