[alpha] misc unwinding changes

[Roland McGrath]

> And to do all this, I require CFI assembler directives added to
> gas only this morning, thus the subject of the generic/sysdep.h change.
>
> Personally I'm fine with requiring today's assembler to build.
> Does anyone have any objections to this?  The result without is
> demonstrably incorrect...

This is ok with me as long as you update the configure check so that it
doesn't define HAVE_ASM_CFI_DIRECTIVES for yesterday's assembler.