compiler_types.h: fix "unused variable" in __compiletime_assert()

This refines commit c03567a8e8d5 ("include/linux/compiler.h: don't perform
compiletime_assert with -O0") and restores #ifdef __OPTIMIZE__ symmetry by
evaluating the 'condition' variable in both compile-time variants of
__compiletimeassert().

As __OPTIMIZE__ is always true by default, this commit does not change
anything by default. But it fixes warnings with _non-default_ CFLAGS like
for instance this:

make CFLAGS_tcp.o='-Og -U__OPTIMIZE__'

from net/ipv4/tcp.c:273:

include/net/sch_generic.h: In function `qdisc_cb_private_validate':
include/net/sch_generic.h:511:30:
error: unused variable `qcb' [-Werror=unused-variable]

{
struct qdisc_skb_cb *qcb;

BUILD_BUG_ON(sizeof(skb->cb) < sizeof(*qcb));
...
}

[akpm@linux-foundation.org: regularize comment layout, reflow comment]
Link: https://lkml.kernel.org/r/20250424194048.652571-1-marc.herbert@linux.intel.com
Signed-off-by: Marc Herbert <Marc.Herbert@linux.intel.com>
Cc: Alexander Potapenko <glider@google.com>
Cc: Changbin Du <changbin.du@intel.com>
Cc: Jan Hendrik Farr <kernel@jfarr.cc>
Cc: Macro Elver <elver@google.com>
Cc: Miguel Ojeda <ojeda@kernel.org>
Cc: Tony Ambardar <tony.ambardar@gmail.com>
Cc: Uros Bizjak <ubizjak@gmail.com>
Cc: Kees Cook <kees@kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>