[SYCL] Fix __builtin_sycl_unique_stable_name to work on windows/spir

In the case where the device is an itanium target, and the host is a
windows target, we were getting the names wrong, since in the itanium
case we filter by lambda-signature.

The fix is to always filter by the signature rather than just on
non-windows builds. I considered doing the reverse (that is, checking
the aux-triple), but doing so would result in duplicate lambda mangling
numbers (from linux reusing the same number for different signatures).