[PATCH] Build libgcc_s on Windows

[Aaron W. LaFramboise]

This patch enables building libgcc_s as a DLL on MinGW and Cygwin.  This 
allows throwing exceptions across DLL boundaries when using a shared 
libgcc_s.

It needs approval from toplevel configure, gcc/mkmap, Windows and 
libstdc++ maintainers.

It was tested by a bootstrap and testsuite run on i386-pc-mingw32, as 
well as manually to make sure it works.

The mkmap machinery is re-used to generate a .def file, which is PECOFF 
equivalent of a flat map file.  This should help control the ABI a little.

OK to commit?
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