loongarch: Use ASM_REACHABLE

annotate_reachable() is unreliable since the compiler is free to place
random code inbetween two consecutive asm() statements.

This removes the last and only annotate_reachable() user.

Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Acked-by: Josh Poimboeuf <jpoimboe@kernel.org>
Link: https://lore.kernel.org/r/20241128094312.133437051@infradead.org