selftests/bpf: Honor $(O) when figuring out paths

When building the kselftests out-of-tree, e.g. ...

  | make ARCH=riscv CROSS_COMPILE=riscv64-linux-gnu- \
  |   O=/tmp/kselftest headers
  | make ARCH=riscv CROSS_COMPILE=riscv64-linux-gnu- \
  |  O=/tmp/kselftest HOSTCC=gcc FORMAT= \
  |  SKIP_TARGETS="arm64 ia64 powerpc sparc64 x86 sgx" \
  |  -C tools/testing/selftests gen_tar

... the kselftest build would not pick up the correct GENDIR path, and
therefore not including autoconf.h.

Correct that by taking $(O) into consideration when figuring out the
GENDIR path.

Signed-off-by: Björn Töpel <bjorn@rivosinc.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Link: https://lore.kernel.org/bpf/20230705113926.751791-3-bjorn@kernel.org