[UR] Allow urProgramGetFunctionPointer to return UNSUPPORTED

[RossBrunton]

This is primarily for liboffload which doesn't support it yet, and
probably never will for AMD devices (which don't seem to have this
functionality). In theory it will also affect level zero, at least
according to a comment, but urProgramGetFunctionPointer seems to be
implemented there.

[RossBrunton]

Also, this is nonsensical error message (functions are not kernels). But that's probably a change to do elsewhere.

[callumfare]

It doesn't look like the SYCL runtime uses this function at all, do we still need it?

[RossBrunton]

@callumfare I think it was added to support calling function pointers from device code, but all information about that has apparently been purged from the internet.

Still, if nobody uses it, then that's all the more reason to make it an optional feature. :P

[RossBrunton]

@intel/llvm-gatekeepers Please merge.

@intel/unified-runtime-reviewers-level-zero This change doesn't touch any Level Zero related code, however it does change a test which indicated that L0 was misbehaving. This change makes the "misbehaving" behaviour valid. Just pinging as an FYI.