libbpf: Fix accessing the first syscall argument on s390

On s390, the first syscall argument should be accessed via orig_gpr2
(see arch/s390/include/asm/syscall.h). Currently gpr[2] is used
instead, leading to bpf_syscall_macro test failure.

Reported-by: Andrii Nakryiko <andrii@kernel.org>
Signed-off-by: Ilya Leoshkevich <iii@linux.ibm.com>
Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Link: https://lore.kernel.org/bpf/20220204145018.1983773-12-iii@linux.ibm.com

Author: iii-i

tools/lib/bpf/bpf_tracing.h

@@ -117,6 +117,7 @@
 /* s390 provides user_pt_regs instead of struct pt_regs to userspace */
 #define __PT_REGS_CAST(x) ((const user_pt_regs *)(x))
 #define __PT_PARM1_REG gprs[2]
+#define __PT_PARM1_REG_SYSCALL orig_gpr2
 #define __PT_PARM2_REG gprs[3]
 #define __PT_PARM3_REG gprs[4]
 #define __PT_PARM4_REG gprs[5]