bpf: Limit static tcp-cc functions in the .BTF_ids list to x86

During the discussion in [0]. It was pointed out that static functions
in ppc64 is prefixed with ".". For example, the 'readelf -s vmlinux.ppc':

  89326: c000000001383280    24 NOTYPE  LOCAL  DEFAULT   31 cubictcp_init
  89327: c000000000c97c50   168 FUNC    LOCAL  DEFAULT    2 .cubictcp_init

The one with FUNC type is ".cubictcp_init" instead of "cubictcp_init".
The "." seems to be done by arch/powerpc/include/asm/ppc_asm.h.

This caused that pahole cannot generate the BTF for these tcp-cc kernel
functions because pahole only captures the FUNC type and "cubictcp_init"
is not. It then failed the kernel compilation in ppc64.

This behavior is only reported in ppc64 so far. I tried arm64, s390,
and sparc64 and did not observe this "." prefix and NOTYPE behavior.

Since the kfunc call is only supported in the x86_64 and x86_32 JIT,
this patch limits those tcp-cc functions to x86 only to avoid unnecessary
compilation issue in other ARCHs. In the future, we can examine if it
is better to change all those functions from static to extern.

  [0] https://lore.kernel.org/bpf/4e051459-8532-7b61-c815-f3435767f8a0@kernel.org/

Fixes: e78aea8b ("bpf: tcp: Put some tcp cong functions in allowlist for bpf-tcp-cc")
Signed-off-by: Martin KaFai Lau <kafai@fb.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Cc: Michal Suchánek <msuchanek@suse.de>
Cc: Jiri Slaby <jslaby@suse.com>
Cc: Jiri Olsa <jolsa@redhat.com>
Link: https://lore.kernel.org/bpf/20210508005011.3863757-1-kafai@fb.com

net/ipv4/bpf_tcp_ca.c

@@ -185,6 +185,7 @@ BTF_ID(func, tcp_reno_cong_avoid)
 BTF_ID(func, tcp_reno_undo_cwnd)
 BTF_ID(func, tcp_slow_start)
 BTF_ID(func, tcp_cong_avoid_ai)
+#ifdef CONFIG_X86
 #ifdef CONFIG_DYNAMIC_FTRACE
 #if IS_BUILTIN(CONFIG_TCP_CONG_CUBIC)
 BTF_ID(func, cubictcp_init)
@@ -213,6 +214,7 @@ BTF_ID(func, bbr_min_tso_segs)
 BTF_ID(func, bbr_set_state)
 #endif
 #endif  /* CONFIG_DYNAMIC_FTRACE */
+#endif	/* CONFIG_X86 */
 BTF_SET_END(bpf_tcp_ca_kfunc_ids)
 
 static bool bpf_tcp_ca_check_kfunc_call(u32 kfunc_btf_id)