bug: Fix no-return-statement warning with !CONFIG_BUG

bug: Fix no-return-statement warning with !CONFIG_BUG

BUG() does not return, and arch implementations of BUG() use unreachable()
or other non-returning code. However with !CONFIG_BUG, the default
implementation is often used instead, and that does not do that. x86 always
uses its own implementation, but powerpc with !CONFIG_BUG gives a build
error:

  kernel/time/timekeeping.c: In function ‘timekeeping_debug_get_ns’:
  kernel/time/timekeeping.c:286:1: error: no return statement in function
  returning non-void [-Werror=return-type]

Add unreachable() to default !CONFIG_BUG BUG() implementation.

Fixes: e8e9d21a5df6 ("timekeeping: Refactor timekeeping helpers")
Reported-by: Naresh Kamboju <naresh.kamboju@linaro.org>
Signed-off-by: Adrian Hunter <adrian.hunter@intel.com>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Tested-by: Linux Kernel Functional Testing <lkft@linaro.org>
Link: https://lore.kernel.org/r/20240410153212.127477-1-adrian.hunter@intel.com
Closes: https://lore.kernel.org/all/CA+G9fYvjdZCW=7ZGxS6A_3bysjQ56YF7S-+PNLQ_8a4DKh1Bhg@mail.gmail.com/