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-tx.cpp

@@ -8,6 +8,7 @@
 
 #include <clientversion.h>
 #include <coins.h>
+#include <compat.h>
 #include <consensus/amount.h>
 #include <consensus/consensus.h>
 #include <core_io.h>
@@ -854,7 +855,7 @@ static int CommandLineRawTx(int argc, char* argv[])
     return nRet;
 }
 
-int main(int argc, char* argv[])
+MAIN_FUNCTION
 {
     SetupEnvironment();