Support std::memcpy or improve detail::memcpy

[Pennycook]

Is your feature request related to a problem? Please describe
As noted in #3815, the performance behavior of sycl::detail::memcpy is different to the performance behavior of std::memcpy. In my tests, performance is up to 2x better with std::memcpy.

Describe the solution you would like
I think there are two options:

1. Support std::memcpy in device code.
   This appears to work already, but the function isn't explicitly listed here. This is my preferred solution, because it would allow us to call std::memcpy in the implementation and for users to call std::memcpy in their kernels.

2. Implement sycl::detail::memcpy the same way as std::memcpy where possible.
   The implementation of sycl::detail::memcpy here is just a simple loop, and the compiler doesn't seem to optimize this as aggressively as it does std::memcpy. Making sycl::detail::memcpy faster wouldn't help user code, but would improve performance for those parts of the implementation currently relying on it.

Describe alternatives you have considered
Calling __builtin_memcpy might also work, but adding a third variant of memcpy to the mix seems more confusing.

Additional context
I think there are other headers that currently assume std::memcpy works in device kernels, and I wouldn't be surprised if there was also user code relying on this behavior.

[bader]

Tagging @jinge90.

[jinge90]

Hi, @bader and @Pennycook
We also plan to add support std::memcmp, std::memcpy in SYCL device library since we found some std items request by users had dependency on those mem* functions.
Thanks very much.

[Pennycook]

Thanks, @jinge90. Do we have an ETA?

@bader: All of the tests in #3815 pass. Would you still rather wait until std::memcpy is officially supported before merging?

[bader]

All of the tests in #3815 pass. Would you still rather wait until std::memcpy is officially supported before merging?

I'm not sure if CI pre-commit tests cover shuffle functionality very well, but if you okay to address issue found post-commit, I don't have any objections.

[Pennycook]

Closed by #3879.