libbpf: Use struct user_pt_regs to define __PT_REGS_CAST() for LoongArch

LoongArch provides struct user_pt_regs instead of struct pt_regs
to userspace, use struct user_pt_regs to define __PT_REGS_CAST()
to fix the following build error:

     CLNG-BPF [test_maps] loop1.bpf.o
  progs/loop1.c:22:9: error: incomplete definition of type 'struct pt_regs'
                                  m = PT_REGS_RC(ctx);
                                      ^~~~~~~~~~~~~~~
  tools/testing/selftests/bpf/tools/include/bpf/bpf_tracing.h:493:41: note: expanded from macro 'PT_REGS_RC'
  #define PT_REGS_RC(x) (__PT_REGS_CAST(x)->__PT_RC_REG)
                         ~~~~~~~~~~~~~~~~~^
  tools/testing/selftests/bpf/tools/include/bpf/bpf_helper_defs.h:20:8: note: forward declaration of 'struct pt_regs'
  struct pt_regs;
         ^
  1 error generated.
  make: *** [Makefile:572: tools/testing/selftests/bpf/loop1.bpf.o] Error 1
  make: Leaving directory 'tools/testing/selftests/bpf'
Signed-off-by: Tiezhu Yang <yangtiezhu@loongson.cn>
Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Link: https://lore.kernel.org/bpf/1677235015-21717-2-git-send-email-yangtiezhu@loongson.cn

tools/lib/bpf/bpf_tracing.h

@@ -415,6 +415,8 @@ struct pt_regs___arm64 {
  * https://loongson.github.io/LoongArch-Documentation/LoongArch-ELF-ABI-EN.html
  */
 
+/* loongarch provides struct user_pt_regs instead of struct pt_regs to userspace */
+#define __PT_REGS_CAST(x) ((const struct user_pt_regs *)(x))
 #define __PT_PARM1_REG regs[4]
 #define __PT_PARM2_REG regs[5]
 #define __PT_PARM3_REG regs[6]