SYCL: Add table of GNU Make config vars to docs

[mirenradia]

Summary

This PR allows the user to parallelize the AOT device compilation when building with the SYCL backend using GNU Make.
This PR adds a table of GNU make config vars to the docs.

Additional background

When building with AOT device compilation on the SYCL backend, the device compilation takes place at the linking step and so is not currently parallelized (even when leveraging multiple GNU Make jobs e.g. by using the -j flag). It can therefore be quite slow. This PR introduces the SYCL_MAX_PARALLEL_LINK_JOBS makefile variable which is passed as the argument to the -fsycl-max-parallel-link-jobs flag of icpx. Although the Intel documentation does state

It is not guaranteed that spawning device link processes can be safely combined with the build system-level parallelization.

I think it's probably fine for the case of building an AMReX application where there is only 1 link step at the end after compiling source files.

To accompany this change, I have added a table of GNU make variables that are relevant to SYCL.

I guess it would make sense to add a similar option to the CMake build system but I am not very familiar with it.

Checklist

The proposed changes:

• fix a bug or incorrect behavior in AMReX
• add new capabilities to AMReX
• changes answers in the test suite to more than roundoff level
• are likely to significantly affect the results of downstream AMReX users
• include documentation in the code and/or rst files, if appropriate

[WeiqunZhang]

Thanks! I will do follow-up on CMake.

[mirenradia]

I made the changes you requested and updated the name and original description of the PR.