selftests/bpf: Use PT_REGS_SYSCALL_REGS in bpf_syscall_macro

Ensure that PT_REGS_SYSCALL_REGS works correctly.

Signed-off-by: Ilya Leoshkevich <iii@linux.ibm.com>
Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Link: https://lore.kernel.org/bpf/20220209021745.2215452-4-iii@linux.ibm.com