For kernel builds, we redefine __printf__ to __freebsd_kprintf__, to
support FreeBSD kernel printf(9) extensions with clang.

In OpenZFS various printf related functions are declared with
attribute((format(printf, X, Y))), so these won't work with the
above redefinition. With clang 21 and higher, this leads to errors
similar to:

sys/contrib/openzfs/module/zfs/spa_misc.c:414:38: error: passing 'printf' format string where 'freebsd_kprintf' format string is expected [-Werror,-Wformat]
  414 |         (void) vsnprintf(buf, sizeof (buf), fmt, adx);
      |                                             ^

Since attribute names can always be spelled with leading and trailing
double underscores, rename these instances.

Note that in FreeBSD proper we usually use __printflike from
<sys/cdefs.h>, but that does not apply to OpenZFS.

MFC after: 3 days