mingw: improve crash information reporting
Up to now we have always released libsipe.dll without stripping it. But
Pidgin crash reporter expects files with debug symbols at a special
location.
Update the MinGW build to also generate & install a stripped
libsipe.dll, just like Pidgin Windows does.
Update the NSIS installer to copy the stripped libsipe.dll to the usual
place and the non-stripped libsipe.dll.dbgsym into the same location
where Pidgin expects files with debug symbols.
The Pidgin.RPT file should now contain more information when SIPE causes
a crash.
(cherry picked from commit
528db6a9b4015f6653c4b6681f43d764b8bbc8a3)