add cmake options to disable building tests and to use llvm linker

[rem1776]

Is your feature request related to a problem? Please describe.
building the tests should not be required always, and intel oneapi compielers sometimes requires a different linker depending on your system

Describe the solution you'd like
add options to CMakeLists.txt

Describe alternatives you've considered

Additional context
#1809

[mathomp4]

@rem1776 This didn't get into 2026.01, right? Just want to be sure

[rem1776]

@mathomp4 No this isn't in yet unfortunately. I'll make sure we include this in the next release.

[mathomp4]

@rem1776 No worries. I was going to do some Spack and Baselibs hacking soon and wanted to make sure it wasn't wasted effort :-)

[fmalatino]

I plan on partially addressing this issue in PR 1850