[avr] fixup dwarf2 unwind info

[Richard Henderson]

This is a slightly tidied patch from sometime last month, which was
buried in a thread about dwarf2 unwind info for gas.  This fixes 
some totally incorrect unwind markings in the avr backend, and adds
support in the dwarf2 middle-end for handling avr's unique post_dec
(rather than pre_dec) push insns.

Ok to commit?


r~
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