[llvm] [OFFLOAD] Add level_zero to list of default plugins (PR #210070)

[Alex Duran via llvm-commits]

adurang wrote:

Passrates for unittests are well 90%, OpenMP tests are also above 70% (and most issues are related to spirv and not really the plugin) so as it has matured substantially from the inital upstream code, it's time to add it to the default list as more people are starting to use it.

https://github.com/llvm/llvm-project/pull/210070