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/qt/main.cpp

@@ -4,6 +4,7 @@
 
 #include <qt/bitcoin.h>
 
+#include <compat.h>
 #include <util/translation.h>
 #include <util/url.h>
 
@@ -18,4 +19,7 @@ extern const std::function<std::string(const char*)> G_TRANSLATION_FUN = [](cons
 };
 UrlDecodeFn* const URL_DECODE = urlDecode;
 
-int main(int argc, char* argv[]) { return GuiMain(argc, argv); }
+MAIN_FUNCTION
+{
+    return GuiMain(argc, argv);
+}