selftests/bpf: Fix data_in_arena test with GCC

GCC doesn't support indirect calls in BPF:

  progs/data_in_arena.c: In function 'use_ops':
  progs/data_in_arena.c:65:1: error: indirect call in function, which are not supported by eBPF

Call add1() directly when the test is built with GCC. use_ops() still
loads the pointers to data that are next to the pointer to the function
in .data.rel.ro and dereferences them.

Once the build is fixed data_in_arena/arena_var_no_map fails with GCC:

  test_arena_var_no_map:FAIL:open unexpected pointer: 0x559fb35d3ca0

GCC doesn't support address spaces. __arena is a type tag only there,
so avar is in .data instead of .addr_space.1 and there is nothing wrong
with the object. Use __arena_global. It puts the variable into
.addr_space.1 with both compilers.

Fixes: 27e24617a8 ("selftests/bpf: Add test for global data in arena")
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Link: https://lore.kernel.org/bpf/20261003150456.168903-1-alexei.starovoitov@gmail.com
Signed-off-by: Kumar Kartikeya Dwivedi <memxor@gmail.com>

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

@@ -61,7 +61,12 @@ int use_ops(void *ctx)
 {
 	/* a program has the arena when its code refers to it */
 	counter++;
+#ifdef __clang__
 	return ops.fn(*ops.data) + ops.name[1];
+#else
+	/* gcc doesn't support indirect calls */
+	return add1(*ops.data) + ops.name[1];
+#endif
 }
 
 SEC("syscall")

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

@@ -9,7 +9,7 @@ char data_in_arena SEC(".arena.data");
 
 int counter = 5;
 /* needs an arena map that is declared. The one that libbpf creates won't do. */
-int __arena avar = 11;
+int __arena_global avar = 11;
 
 SEC("syscall")
 int arena_var(void *ctx)