LoongArch: Move __arch_cpu_idle() to .cpuidle.text section

Now arch_cpu_idle() is annotated with __cpuidle which means it is in
the .cpuidle.text section, but __arch_cpu_idle() isn't. Thus, fix the
missing .cpuidle.text section assignment for __arch_cpu_idle() in order
to correct backtracing with nmi_backtrace().

The principle is similar to the commit 97c8580e ("MIPS: Annotate
cpu_wait implementations with __cpuidle")

Cc: stable@vger.kernel.org
Signed-off-by: Huacai Chen <chenhuacai@loongson.cn>