libbpf: Add LoongArch support to bpf_tracing.h

Add PT_REGS macros for LoongArch ([0]).

  [0]: https://loongson.github.io/LoongArch-Documentation/LoongArch-ELF-ABI-EN.html

Signed-off-by: Hengqi Chen <hengqi.chen@gmail.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Acked-by: Huacai Chen <chenhuacai@loongson.cn>
Link: https://lore.kernel.org/bpf/20221231100757.3177034-1-hengqi.chen@gmail.com