selftests/bpf: sync kernel headers and introduce arch support in Makefile

Synchronize the uapi kernel header files which solves the broken
uapi export of pt_regs. Because of arch-specific uapi headers,
extended the include path in the Makefile.

With this change, the test_verifier program compiles and runs successfully
on s390.

Signed-off-by: Hendrik Brueckner <brueckner@linux.vnet.ibm.com>
Reviewed-and-tested-by: Thomas Richter <tmricht@linux.vnet.ibm.com>
Acked-by: Alexei Starovoitov <ast@kernel.org>
Cc: Daniel Borkmann <daniel@iogearbox.net>
Cc: Shuah Khan <shuah@kernel.org>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>