bpf: adapt selftests to GCC 16 -Wunused-but-set-variable

GCC 16 has changed the semantics of -Wunused-but-set-variable, as well
as introducing new options -Wunused-but-set-variable={0,1,2,3} to
adjust the level of support.

One of the changes is that GCC now treats 'sum += 1' and 'sum++' as
non-usage, whereas clang (and GCC < 16) considers the first as usage
and the second as non-usage, which is sort of inconsistent.

The GCC 16 -Wunused-but-set-variable=2 option implements the previous
semantics of -Wunused-but-set-variable, but since it is a new option,
it cannot be used unconditionally for forward-compatibility, just for
backwards-compatibility.

So this patch adds pragmas to the two self-tests impacted by this,
progs/free_timer.c and progs/rcu_read_lock.c, to make gcc to ignore
-Wunused-but-set-variable warnings when compiling them with GCC > 15.

See https://gcc.gnu.org/bugzilla/show_bug.cgi?id=44677#c25 for details
on why this regression got introduced in GCC upstream.

Signed-off-by: Jose E. Marchesi <jose.marchesi@oracle.com>
Cc: david.faust@oracle.com
Cc: cupertino.miranda@oracle.com
Cc: Eduard Zingerman <eddyz87@gmail.com>
Cc: Yonghong Song <yonghong.song@linux.dev>
Link: https://lore.kernel.org/r/20260106173650.18191-2-jose.marchesi@oracle.com
Signed-off-by: Alexei Starovoitov <ast@kernel.org>

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