libbpf: Add BPF_KPROBE_SYSCALL macro

Add syscall-specific variant of BPF_KPROBE named BPF_KPROBE_SYSCALL ([0]).
The new macro hides the underlying way of getting syscall input arguments.
With the new macro, the following code:

    SEC("kprobe/__x64_sys_close")
    int BPF_KPROBE(do_sys_close, struct pt_regs *regs)
    {
        int fd;

        fd = PT_REGS_PARM1_CORE(regs);
        /* do something with fd */
    }

can be written as:

    SEC("kprobe/__x64_sys_close")
    int BPF_KPROBE_SYSCALL(do_sys_close, int fd)
    {
        /* do something with fd */
    }

  [0] Closes: https://github.com/libbpf/libbpf/issues/425



Signed-off-by: Hengqi Chen <hengqi.chen@gmail.com>
Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Link: https://lore.kernel.org/bpf/20220207143134.2977852-2-hengqi.chen@gmail.com