target/loongarch: Fix qemu-system-loongarch64 assert failed with the option '-d int'

target/loongarch: Fix qemu-system-loongarch64 assert failed with the option '-d int'

qemu-system-loongarch64 assert failed with the option '-d int',
the helper_idle() raise an exception EXCP_HLT, but the exception name is undefined.

Signed-off-by: Song Gao <gaosong@loongson.cn>
Reviewed-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Message-Id: <20240321123606.1704900-1-gaosong@loongson.cn>
(cherry picked from commit 1590154ee4376819a8c6ee61e849ebf4a4e7cd02)
Signed-off-by: Michael Tokarev <mjt@tls.msk.ru>