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/bitcoin-util.cpp

@@ -11,6 +11,7 @@
 #include <chainparams.h>
 #include <chainparamsbase.h>
 #include <clientversion.h>
+#include <compat.h>
 #include <core_io.h>
 #include <streams.h>
 #include <util/system.h>
@@ -142,16 +143,7 @@ static int Grind(const std::vector<std::string>& args, std::string& strPrint)
     return EXIT_SUCCESS;
 }
 
-#ifdef WIN32
-// Export main() and ensure working ASLR on Windows.
-// Exporting a symbol will prevent the linker from stripping
-// the .reloc section from the binary, which is a requirement
-// for ASLR. This is a temporary workaround until a fixed
-// version of binutils is used for releases.
-__declspec(dllexport) int main(int argc, char* argv[])
-#else
-int main(int argc, char* argv[])
-#endif
+MAIN_FUNCTION
 {
     ArgsManager& args = gArgs;
     SetupEnvironment();