selftests/bpf: Fix build of task_pt_regs test for arm64

struct pt_regs is not exported to userspace on all archs. arm64 and s390
export "user_pt_regs" instead, which causes build failure at the moment:

  progs/test_task_pt_regs.c:8:16: error: variable has incomplete type 'struct pt_regs'
  struct pt_regs current_regs = {};

Instead of using pt_regs from ptrace.h, use the larger kernel struct
from vmlinux.h directly. Since the test runner task_pt_regs.c does not
have access to the kernel struct definition, copy it into a char array.

Fixes: 576d47bb ("bpf: selftests: Add bpf_task_pt_regs() selftest")
Suggested-by: Andrii Nakryiko <andrii.nakryiko@gmail.com>
Signed-off-by: Jean-Philippe Brucker <jean-philippe@linaro.org>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Tested-by: Ilya Leoshkevich <iii@linux.ibm.com>
Acked-by: Ilya Leoshkevich <iii@linux.ibm.com>
Link: https://lore.kernel.org/bpf/20210906163635.302307-1-jean-philippe@linaro.org

tools/testing/selftests/bpf/prog_tests/task_pt_regs.c

 // SPDX-License-Identifier: GPL-2.0
 #define _GNU_SOURCE
 #include <test_progs.h>
-#include <linux/ptrace.h>
 #include "test_task_pt_regs.skel.h"
 
 void test_task_pt_regs(void)

tools/testing/selftests/bpf/progs/test_task_pt_regs.c

 // SPDX-License-Identifier: GPL-2.0
 
-#include <linux/ptrace.h>
-#include <linux/bpf.h>
+#include "vmlinux.h"
 #include <bpf/bpf_helpers.h>
 #include <bpf/bpf_tracing.h>
 
-struct pt_regs current_regs = {};
-struct pt_regs ctx_regs = {};
+#define PT_REGS_SIZE sizeof(struct pt_regs)
+
+/*
+ * The kernel struct pt_regs isn't exported in its entirety to userspace.
+ * Pass it as an array to task_pt_regs.c
+ */
+char current_regs[PT_REGS_SIZE] = {};
+char ctx_regs[PT_REGS_SIZE] = {};
 int uprobe_res = 0;
 
 SEC("uprobe/trigger_func")
@@ -17,8 +22,10 @@ int handle_uprobe(struct pt_regs *ctx)
 
 	current = bpf_get_current_task_btf();
 	regs = (struct pt_regs *) bpf_task_pt_regs(current);
-	__builtin_memcpy(&current_regs, regs, sizeof(*regs));
-	__builtin_memcpy(&ctx_regs, ctx, sizeof(*ctx));
+	if (bpf_probe_read_kernel(current_regs, PT_REGS_SIZE, regs))
+		return 0;
+	if (bpf_probe_read_kernel(ctx_regs, PT_REGS_SIZE, ctx))
+		return 0;
 
 	/* Prove that uprobe was run */
 	uprobe_res = 1;