[PATCH v2 bpf-next 0/4] selftests/bpf: fix compiling loop{1,2,3}.c on s390

[Ilya Leoshkevich <iii@xxxxxxxxxxxxx>]

Use PT_REGS_RC(ctx) instead of ctx->rax, which is not present on s390.

This patch series consists of preparatory commits, which make it
possible to use PT_REGS_RC in BPF selftests, followed by the actual fix.

Signed-off-by: Ilya Leoshkevich <iii@xxxxxxxxxxxxx>