CMake: AMReX_PARALLEL_LINK_JOBS

[WeiqunZhang]

Add AMReX_PARALLEL_LINK_JOBS option to the CMake build system and use it for SYCL builds.

Adjust SYCL's RelWithDebInfo mode to the recommendation of Intel VTune.

This is a follow-up on #3498 that made similar changes to GNU Make.

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

[ax3l]

Great option!

I think there is a general option now for vanilla LLVM and GCC's LTO as well that supports threading... Maybe we can unify them into a general name instead of calling it SYCL?

AMReX_SYCL_PARALLEL_LINK_JOBS -> AMReX_PARALLEL_LINK_JOBS?

[ax3l]

We have to be careful not to oversubscribe CI:
if we build more than one AMReX lib (1D, 2D, 3D), then CMake will with --parallel 2 already run two linkers.

This is not the case here, so we are good 👍

[WeiqunZhang]

AMReX_SYCL_PARALLEL_LINK_JOBS -> AMReX_PARALLEL_LINK_JOBS?

Sounds good. I will make the change. But will limit its use to SYCL for now.