[RFC/Patch] : Enable Dwarf2 unwind on cygwin and mingw32

[Andrew Haley]

Danny Smith writes:
 > 
 > I have been using something very similar (sans .jcr) to this in my
 > "personal" builds of gcc since 3.2 days, but I do not use win32 API
 > functions that require callbacks in conjunction with C++ EH, so I
 > have not been hit by the problems with callbacks that at least one
 > mingw user has reported. The recent binutils patches to make weak
 > symbols work on PECOFF means that another objection to Dwarf2 on
 > mingw user list - unnecessary code bloat in absence of exception
 > handling -- can now be avoided.

Don't we need a MD_FALLBACK_FRAME_STATE_FOR to unwind through SEH
regions?  Plent of people do use callbacks AFAIAA.

Andrew.