Fix register naming in PT_REGS s390 macros

Fixes: b8ebce86 ("libbpf: Provide CO-RE variants of PT_REGS macros")
Signed-off-by: NSumanth Korikkar <sumanthk@linux.ibm.com>
Signed-off-by: NAlexei Starovoitov <ast@kernel.org>
Reviewed-by: NJulian Wiedmann <jwi@linux.ibm.com>
Acked-by: NAndrii Nakryiko <andriin@fb.com>
Link: https://lore.kernel.org/bpf/20200513154414.29972-1-sumanthk@linux.ibm.com