selftests/bpf: Guard addr_space_cast code with __BPF_FEATURE_ADDR_SPACE_CAST

When compiling the BPF selftests with Clang versions that do not support
the addr_space_cast builtin, the build fails with assembly errors in
"verifier_ldsx.c" [1].

The root cause is that the inline assembly using addr_space_cast is
being processed by a compiler that lacks this feature. To resolve this,
wrap the affected code sections (specifically the arena_ldsx_* test
functions) with #if defined(__BPF_FEATURE_ADDR_SPACE_CAST). This
ensures the code is only compiled when the Clang supports the necessary
feature, preventing build failures on older or incompatible compiler
versions.

This change maintains test coverage for systems with support while
allowing the tests to build successfully in all environments.

[1]:
root:tools/testing/selftests/bpf$ make

  CLNG-BPF [test_progs] verifier_ldsx.bpf.o
progs/verifier_ldsx.c:322:2: error: invalid operand for instruction
  322 |         "r1 = %[arena] ll;"
      |         ^
<inline asm>:1:52: note: instantiated into assembly here
    1 |         r1 = arena ll;r0 = 0xdeadbeef;r0 = addr_space_cast(r0,...
      |                                                           ^

Fixes: f616549 ("selftests: bpf: Add tests for signed loads from arena")
Signed-off-by: Jiayuan Chen <[email protected]>

Author: mrpre

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

@@ -263,6 +263,7 @@ __naked void ldsx_ctx_8(void)
 	: __clobber_all);
 }
 
+#if defined(__BPF_FEATURE_ADDR_SPACE_CAST)
 SEC("syscall")
 __description("Arena LDSX Disasm")
 __success
@@ -425,6 +426,7 @@ __naked void arena_ldsx_s32(void *ctx)
 	:  __clobber_all
 	);
 }
+#endif
 
 /* to retain debug info for BTF generation */
 void kfunc_root(void)