Restore r31 setting in powerpc32 swapcontext

[Florian Weimer]

* Joseph Myers:

> diff --git a/sysdeps/unix/sysv/linux/powerpc/powerpc32/swapcontext-common.S b/sysdeps/unix/sysv/linux/powerpc/powerpc32/swapcontext-common.S
> index efebb10bba..6fa1ab7d6e 100644
> --- a/sysdeps/unix/sysv/linux/powerpc/powerpc32/swapcontext-common.S
> +++ b/sysdeps/unix/sysv/linux/powerpc/powerpc32/swapcontext-common.S
> @@ -276,6 +276,9 @@ ENTRY(__CONTEXT_FUNC_NAME)
>  	cmpwi	r3,0
>  	bne	3f	/* L(error_exit) */
>  
> +	lwz	r4,_FRAME_PARM_SAVE2(r1)
> +	lwz	r31,_UC_REGS_PTR(r4)
> +
>  #ifdef __CONTEXT_ENABLE_FPRS
>  # ifdef __CONTEXT_ENABLE_VRS

Patch looks good to me.  Carlos will have to ack it as the RM, though.

Thanks,
Florian