bpf: fix broken BPF selftest build

At least on x86_64, the kernel's BPF selftests seemed to have stopped
to build due to 618e165b ("selftests/bpf: sync kernel headers and
introduce arch support in Makefile"):

  [...]
  In file included from test_verifier.c:29:0:
  ../../../include/uapi/linux/bpf_perf_event.h:11:32:
     fatal error: asm/bpf_perf_event.h: No such file or directory
   #include <asm/bpf_perf_event.h>
                                ^
  compilation terminated.
  [...]

While pulling in tools/arch/*/include/uapi/asm/bpf_perf_event.h seems
to work fine, there's no automated fall-back logic right now that would
do the same out of tools/include/uapi/asm-generic/bpf_perf_event.h. The
usual convention today is to add a include/[uapi/]asm/ equivalent that
would pull in the correct arch header or generic one as fall-back, all
ifdef'ed based on compiler target definition. It's similarly done also
in other cases such as tools/include/asm/barrier.h, thus adapt the same
here.

Fixes: 618e165b ("...