[libcxx] Mark strstreams tests as being supported on all OS X versions

I wasn't able to reproduce the issue referred to by the comment using
the libc++'s shipped with mac OS X 10.7 and 10.8, so I assume this may
have been fixed in a function that is now shipped in the headers. In
that case, the tests will pass no matter what dylib we're using.

In the worst case, some test bots will start failing and I'll understand
why I was wrong, and I can create an actual lit feature for it. Note
that I could just leave this test alone, but this change is on the path
towards eradicating vendor-specific availability markup from the test
suite.

llvm-svn: 347421