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>

linux-user/loongarch64/signal.c

@@ -115,7 +115,7 @@ static abi_ptr setup_extcontext(struct extctx_layout *extctx, abi_ptr sp)
     /* For qemu, there is no lazy fp context switch, so fp always present. */
     extctx->flags = SC_USED_FP;
     sp = extframe_alloc(extctx, &extctx->fpu,
-                        sizeof(struct target_rt_sigframe), FPU_CTX_ALIGN, sp);
+                        sizeof(struct target_fpu_context), FPU_CTX_ALIGN, sp);
 
     return sp;
 }