selftests/bpf: trace_helpers.c: do not use poisoned type

After commit c698eae ("selftests/bpf: trace_helpers.c: Optimize
kallsyms cache") trace_helpers.c now includes libbpf_internal.h, and
thus can no longer use the u32 type (among others) since they are poison
in libbpf_internal.h. Replace u32 with __u32 to fix the following error
when building trace_helpers.c on powerpc:

  error: attempt to use poisoned "u32"

Fixes: c698eae ("selftests/bpf: trace_helpers.c: Optimize kallsyms cache")
Signed-off-by: Shung-Hsi Yu <shung-hsi.yu@suse.com>
Acked-by: Jiri Olsa <jolsa@kernel.org>
Link: https://lore.kernel.org/r/20240202095559.12900-1-shung-hsi.yu@suse.com
Signed-off-by: Martin KaFai Lau <martin.lau@kernel.org>

tools/testing/selftests/bpf/trace_helpers.c

@@ -271,7 +271,7 @@ ssize_t get_uprobe_offset(const void *addr)
 	 * addi  r2,r2,XXXX
 	 */
 	{
-		const u32 *insn = (const u32 *)(uintptr_t)addr;
+		const __u32 *insn = (const __u32 *)(uintptr_t)addr;
 
 		if ((((*insn & OP_RT_RA_MASK) == ADDIS_R2_R12) ||
 		     ((*insn & OP_RT_RA_MASK) == LIS_R2)) &&