libbpf: Simplify barrier_var()

Use a single "+r" constraint instead of the separate "=r" and "0".
Signed-off-by: Ilya Leoshkevich <iii@linux.ibm.com>
Link: https://lore.kernel.org/r/20230128000650.1516334-22-iii@linux.ibm.comSigned-off-by: Alexei Starovoitov <ast@kernel.org>

tools/lib/bpf/bpf_helpers.h

@@ -109,7 +109,7 @@
  * This is a variable-specific variant of more global barrier().
  */
 #ifndef barrier_var
-#define barrier_var(var) asm volatile("" : "=r"(var) : "0"(var))
+#define barrier_var(var) asm volatile("" : "+r"(var))
 #endif
 
 /*