fix: add PREEMPT_LAZY = mkForce no to resolve choice conflict with PREEMPT

The common-config.nix sets PREEMPT_LAZY = yes for kernels >= 6.18
(which includes 7.1). Both PREEMPT and PREEMPT_LAZY are in the same
preemption model choice group in kernel Kconfig. When generate-config.pl
sees both options with 'y' in the answer file, it dies with
'conflicting answers!' (the "Error in reading or end of file" failure).

Explicitly disabling PREEMPT_LAZY ensures only PREEMPT (full preemption)
is selected, avoiding the choice conflict. Also removes CC_IS_CLANG and
LTO_CLANG_FULL which can't be forced via kernel config with GCC build.