xtensa: add fixup for double exception raised in window overflow

There are two FIXMEs in the double exception handler 'for the extremely
unlikely case'. This case gets hit by gcc during kernel build once in
a few hours, resulting in an unrecoverable exception condition.

Provide missing fixup routine to handle this case. Double exception
literals now need 8 more bytes, add them to the linker script.

Also replace bbsi instructions with bbsi.l as we're branching depending
on 8th and 7th LSB-based bits of exception address.

This may be tested by adding the explicit DTLB invalidation to window
overflow handlers, like the following:

    --- a/arch/xtensa/kernel/vectors.S
    +++ b/arch/xtensa/kernel/vectors.S
    @@ -592,6 +592,14 @@ ENDPROC(_WindowUnderflow4)
     ENTRY_ALIGN64(_WindowOverflow8)

    	s32e	a0, a9, -16
    +	bbsi.l	a9, 31, 1f
    +	rsr	a0, ccount
    +	bbsi.l	a0, 4, 1f
    +	pdtlb	a0, a9
    +	idtlb	a0
    +	movi	a0, 9
    +	idtlb	a0
    +1:
    	l32e    a0, a1, -12
    	s32e    a2, a9,  -8
    	s32e    a1, a9, -12

Cc: stable@vger.kernel.org
Signed-off-by: Max Filippov <jcmvbkbc@gmail.com>