x86/asm: Avoid generating unused kprobe sections

When !CONFIG_KPROBES, do not generate kprobe sections. This makes
sure there are no unexpected sections encountered by the linker scripts.

Signed-off-by: Kees Cook <keescook@chromium.org>
Signed-off-by: Ingo Molnar <mingo@kernel.org>
Link: https://lore.kernel.org/r/20200821194310.3089815-23-keescook@chromium.org