libbpf: Fix accessing first syscall argument on RV64

On RV64, as Ilya mentioned before [0], the first syscall parameter should be
accessed through orig_a0 (see arch/riscv64/include/asm/syscall.h),
otherwise it will cause selftests like bpf_syscall_macro, vmlinux,
test_lsm, etc. to fail on RV64. Let's fix it by using the struct pt_regs
style CO-RE direct access.

Signed-off-by: Pu Lehui <[email protected]>
Signed-off-by: Andrii Nakryiko <[email protected]>
Link: https://lore.kernel.org/bpf/[email protected] [0]
Link: https://lore.kernel.org/bpf/[email protected]

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