LoongArch: Convert unreachable() to BUG()

When compiling on LoongArch, there exists the following objtool warning
in arch/loongarch/kernel/machine_kexec.o:

  kexec_reboot() falls through to next function crash_shutdown_secondary()

Avoid using unreachable() as it can (and will in the absence of UBSAN)
generate fall-through code. Use BUG() so we get a "break BRK_BUG" trap
(with unreachable annotation).

Cc: stable@vger.kernel.org  # 6.12+
Acked-by: Josh Poimboeuf <jpoimboe@kernel.org>
Signed-off-by: Tiezhu Yang <yangtiezhu@loongson.cn>
Signed-off-by: Huacai Chen <chenhuacai@loongson.cn>