compat: Consolidate mingw-w64 ASLR workaround for upstream libsecp changes

Achieve this by adding a MAIN_FUNCTION macro, consolidating the docs, and
introducing the macro across our distributed binaries.

Also update the docs to explain that anyone using binutils < 2.36 is
effected by this issue. Release builds are not, because they use binutils
2.37. Currently LTS Linux distros, like Ubuntu Focal, ship with 2.34.

https://packages.ubuntu.com/focal/binutils

src/bitcoind.cpp

@@ -256,7 +256,7 @@ static bool AppInit(NodeContext& node, int argc, char* argv[])
     return fRet;
 }
 
-int main(int argc, char* argv[])
+MAIN_FUNCTION
 {
 #ifdef WIN32
     util::WinCmdLineArgs winArgs;