[sanitizer_common tests] Make Darwin a Posix system and bring the stable-runtime definition from ASan tests.
Summary: This is an initial setup in order to move some additional tests from Linux onto Posix.
I also moved decorate_proc_maps onto the Linux directory
Finally added msan's definition for "stable-runtime".
Only a test requires it, and its commit message (r248014) seems to imply
that AArch64 is problematic with MSan.
Reviewers: samsonov, rengolin, t.p.northover, eugenis
Differential Revision: http://reviews.llvm.org/D17928