kselftests-bpf: update for kernel 6.6

Enable building multiple test programs and related kernel modules, with
initial support for the bpf_testmod.ko module required since kernel 6.4.
Explicitly disable LTO and clean up makefile variables and formatting.

Fix a musl-related build failure by adding a kernel 6.6 patch:

     360-selftests-bpf-portability-of-unprivileged-tests.patch

Signed-off-by: Tony Ambardar <itugrok@yahoo.com>

target/linux/generic/pending-6.6/360-selftests-bpf-portability-of-unprivileged-tests.patch

@@ -0,0 +1,26 @@
+From ecb8f9a7d69698ce20fc6f4d107718d56fa861df Mon Sep 17 00:00:00 2001
+From: Tony Ambardar <Tony.Ambardar@gmail.com>
+Date: Sat, 9 Mar 2024 16:44:53 -0800
+Subject: [PATCH] selftests/bpf: Improve portability of unprivileged tests
+
+The addition of general support for unprivileged tests in test_loader.c
+breaks building test_verifier on non-glibc (e.g. musl) systems, due to the
+inclusion of glibc extension '<error.h>' in 'unpriv_helpers.c'. However,
+the header is actually not needed, so remove it to restore building.
+
+Fixes: 1d56ade032a4 ("selftests/bpf: Unprivileged tests for test_loader.c")
+Signed-off-by: Tony Ambardar <Tony.Ambardar@gmail.com>
+---
+ tools/testing/selftests/bpf/unpriv_helpers.c | 1 -
+ 1 file changed, 1 deletion(-)
+
+--- a/tools/testing/selftests/bpf/unpriv_helpers.c
++++ b/tools/testing/selftests/bpf/unpriv_helpers.c
+@@ -2,7 +2,6 @@
+ 
+ #include <stdbool.h>
+ #include <stdlib.h>
+-#include <error.h>
+ #include <stdio.h>
+ 
+ #include "unpriv_helpers.h"