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