libbpf: Allow overriding PT_REGS_PARM1{_CORE}_SYSCALL

arm64 and s390 need a special way to access the first syscall argument.

Signed-off-by: Ilya Leoshkevich <iii@linux.ibm.com>
Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Link: https://lore.kernel.org/bpf/20220209021745.2215452-9-iii@linux.ibm.com