[libcxx] Set `_LIBCPP_HAS_CLOCK_GETTIME` for GPU targets (#99243)

Summary:
I am attempting to get the GPU to build and support libc++. One issue
I've encountered is that it will look for `timeval` unless this macro is
set. We can support `CLOCK_MONOTONIC` on the GPU fairly easily as we
have access to a fixed-frequency clock via `__builtin_readsteadycounter`
intrinsics with a known frequency. This also requires `CLOCK_REALTIME`
which we can't support, but provide anyway from the GPU `libc` to make
this happy. It will return an error so at least that will be obvious.

I may need a more consistent configuration for this in the future, maybe
I should put a common macro in a different common header that's just
`__GPU__`? I don't know where I would put such a thing however.

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