[libcxx] [test] Prepare the ctime.timespec test for mingw CI env upgrades

The test is currently marked XFAIL for mingw environments, but latest
mingw-w64 got support for timespec_get:
https://github.com/mingw-w64/mingw-w64/commit/e62a0a987c80f6a6fdac3e350943ae8085de0bd5

The CI environment will probably be upgraded to a state where this
test is passing only after 14.x is branched in the llvm-project monorepo.

If we'd just go from having an XFAIL to no marking at all (when CI is
passing), we'd have to update both main and 14.x branches in sync
exactly when the CI runners are updated to a newer version.

Instead, mark the test as temporarily unsupported (so it doesn't
cause failed builds when the CI environment is updated); after the
CI environments are upgraded to such a state, we can remove the
UNSUPPORTED marking to start requiring it to pass on the main branch,
without needing to synchronize that change to anything else.

Differential Revision: https://reviews.llvm.org/D116132