selftests/bpf: make PT_REGS_* work in userspace

Right now, on certain architectures, these macros are usable only with
kernel headers. This patch makes it possible to use them with userspace
headers and, as a consequence, not only in BPF samples, but also in BPF
selftests.

On s390, provide the forward declaration of struct pt_regs and cast it
to user_pt_regs in PT_REGS_* macros. This is necessary, because instead
of the full struct pt_regs, s390 exposes only its first member
user_pt_regs to userspace, and bpf_helpers.h is used with both userspace
(in selftests) and kernel (in samples) headers. It was added in commit
466698e6 ("s390/bpf: correct broken uapi for
BPF_PROG_TYPE_PERF_EVENT program type").

Ditto on arm64.

On x86, provide userspace versions of PT_REGS_* macros. Unlike s390 and
arm64, x86 provides struct pt_regs to both userspace and kernel, however,
with different member names.

Signed-off-by: Ilya Leoshkevich <iii@linux.ibm.com>
Acked-by: Andrii Nakryiko <andriin@fb.com>
Reviewed-by: Stanislav Fomichev <sdf@google.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>