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-wallet.cpp

@@ -9,6 +9,7 @@
 #include <chainparams.h>
 #include <chainparamsbase.h>
 #include <clientversion.h>
+#include <compat.h>
 #include <interfaces/init.h>
 #include <key.h>
 #include <logging.h>
@@ -88,7 +89,7 @@ static bool WalletAppInit(ArgsManager& args, int argc, char* argv[])
     return true;
 }
 
-int main(int argc, char* argv[])
+MAIN_FUNCTION
 {
     ArgsManager& args = gArgs;
 #ifdef WIN32