[libcxx] Reintroduce UNSUPPORTED annotation for strstreambuf overflow test

This is a revert of r347421, except I'm using the with_system_cxx_lib
lit feature instead of availability to mark the test as unsupported
(because the problem is a bug in the dylib itself). In r347421, I said
I wasn't able to reproduce the issue and that's why I was removing it:
this was because I ran lit slightly wrong. The problem mentioned really
exists.

llvm-svn: 347475