don't build kselftests using gcc for clang built kernels
When building kselftests for clang built kernel we are using gcc, this can cause problems like: https://datawarehouse.cki-project.org/kcidb/tests/11515451
00:05:18 warning: the compiler differs from the one used to build the kernel
00:05:18 The kernel was built by: clang version 17.0.6 (Fedora 17.0.6-6.fc40)
00:05:18 You are using: gcc (GCC) 14.0.1 20240208 (Red Hat 14.0.1-0)
--rethunk --sls --static-call --uaccess --prefix=16 --link --module /builds/6292166687/workdir/tools/testing/selftests/bpf/bpf_testmod/bpf_testmod.o
00:05:18 gcc: error: unrecognized argument to ‘-fsanitize=’ option: ‘array-bounds’
00:05:18 gcc: error: unrecognized command-line option ‘-mretpoline-external-thunk’
Edited by Bruno Goncalves