With an unset CONFIG_CPU_MITIGATIONS, or more precisely with CONFIG_MITIGATION_ITS, I get a compilation error:

ld: arch/x86/net/bpf_jit_comp.o: in function `emit_indirect_jump': bpf_jit_comp.c:(.text+0x8cb): undefined reference to `__x86_indirect_its_thunk_array' make[2]: *** [scripts/Makefile.vmlinux:34: vmlinux] Error 1 make[1]: *** [/usr/src/linux/Makefile:1182: vmlinux] Error 2 make: *** [Makefile:224: __sub-make] Error 2