Fix ASan's Noinst unit tests

We currently skip all "Noinst" unit tests on OS X, which was probably caused when we removed the "allow_reexec" flag. The MaybeReexec function fails to re-execute when the runtime is linked statically, because there is no dylib to use. This patch adds an explicit DisableReexec function that is used from asan_noinst_test.cc and the runtime then doesn't try to re-execute.

Reviewed at http://reviews.llvm.org/D7493

llvm-svn: 228740