linux-user/loongarch64: Fix setup_extcontext alloc wrong fpu_context size

linux-user/loongarch64: Fix setup_extcontext alloc wrong fpu_context size

See:
 https://github.com/torvalds/linux/blob/master/arch/loongarch/kernel/signal.c

The alloc size is sizeof(struct target_fpu_context).

Signed-off-by: Song Gao <gaosong@loongson.cn>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Message-Id: <20231101030816.2353416-3-gaosong@loongson.cn>