[Sanitizers, LSan, Darwin] Allow for lack of VM_MEMORY_OS_ALLOC_ONCE

Summary:
Some time ago, the sanitizers as of r315899 were imported into gcc mainline.  This broke
bootstrap on Darwin 10 and 11, as reported in GCC PR sanitizer/82824
(https://gcc.gnu.org/bugzilla/show_bug.cgi?id=82824) due to the unconditional use
of VM_MEMORY_OS_ALLOC_ONCE.  This was only introduced in Darwin 13/Mac OS X 10.9.

The use of the macro was introduced in r300450.

I couldn't find any statement which Darwin versions are supposed to be supported by
LLVM, but the trivial patch to use the macro only if present allowed the gcc bootstrap
to finish.

So far, I haven't tried building llvm/compiler-rt  on Darwin 11.  Maybe the patch is
simple enough to go in nonetheless.

Committing on behalf of ro.

Reviewers: glider, fjricci, kcc, kuba, kubamracek, george.karpenkov

Reviewed By: fjricci

Subscribers: #sanitizers, zaks.anna, srhines, dberris, kubamracek, llvm-commits

Tags: #sanitizers

Differential Revision: https://reviews.llvm.org/D39888

llvm-svn: 322437