Merge branch 'libbpf: Add syscall-specific variant of BPF_KPROBE'

Hengqi Chen says:

====================

Add new macro BPF_KPROBE_SYSCALL, which provides easy access to syscall
input arguments. See [0] and [1] for background.

  [0]: https://github.com/libbpf/libbpf-bootstrap/issues/57
  [1]: https://github.com/libbpf/libbpf/issues/425

v2->v3:
  - Use PT_REGS_SYSCALL_REGS
  - Move selftest to progs/bpf_syscall_macro.c

v1->v2:
  - Use PT_REGS_PARM2_CORE_SYSCALL instead
====================

Signed-off-by: Andrii Nakryiko <andrii@kernel.org>