selftests/bpf: Fix test_core_reloc_mods on big-endian machines

This is the same as commit d164dd9 ("selftests/bpf: Fix
test_core_autosize on big-endian machines"), but for
test_core_reloc_mods.

Signed-off-by: Ilya Leoshkevich <iii@linux.ibm.com>
Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Link: https://lore.kernel.org/bpf/20211026010831.748682-7-iii@linux.ibm.com

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

@@ -42,7 +42,16 @@ struct core_reloc_mods {
 	core_reloc_mods_substruct_t h;
 };
 
+#if __BYTE_ORDER__ == __ORDER_LITTLE_ENDIAN__
 #define CORE_READ(dst, src) bpf_core_read(dst, sizeof(*(dst)), src)
+#else
+#define CORE_READ(dst, src) ({ \
+	int __sz = sizeof(*(dst)) < sizeof(*(src)) ? sizeof(*(dst)) : \
+						     sizeof(*(src)); \
+	bpf_core_read((char *)(dst) + sizeof(*(dst)) - __sz, __sz, \
+		      (const char *)(src) + sizeof(*(src)) - __sz); \
+})
+#endif
 
 SEC("raw_tracepoint/sys_enter")
 int test_core_mods(void *ctx)