libbpf: Add likely/unlikely macros and use them in selftests

A few selftests and, more importantly, consequent changes to the
bpf_helpers.h file, use likely/unlikely macros, so define them here
and remove duplicate definitions from existing selftests.

Signed-off-by: Anton Protopopov <a.s.protopopov@gmail.com>
Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Link: https://lore.kernel.org/bpf/20250331203618.1973691-3-a.s.protopopov@gmail.com

src/bpf_helpers.h

@@ -15,6 +15,14 @@
 #define __array(name, val) typeof(val) *name[]
 #define __ulong(name, val) enum { ___bpf_concat(__unique_value, __COUNTER__) = val } name
 
+#ifndef likely
+#define likely(x)      (__builtin_expect(!!(x), 1))
+#endif
+
+#ifndef unlikely
+#define unlikely(x)    (__builtin_expect(!!(x), 0))
+#endif
+
 /*
  * Helper macro to place programs, maps, license in
  * different sections in elf_bpf file. Section names