bpf: runqslower: Prefer using local vmlimux to generate vmlinux.h

Update the Makefile to prefer using $(O)/vmlinux, $(KBUILD_OUTPUT)/vmlinux
(for selftests) or ../../../vmlinux. These two files should have latest
definitions for vmlinux.h.

Signed-off-by: Song Liu <songliubraving@fb.com>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Acked-by: Andrii Nakryiko <andrii@kernel.org>
Link: https://lore.kernel.org/bpf/20210225234319.336131-6-songliubraving@fb.com