build: fix ASLR for bitcoin-cli on Windows

ASLR is not currently working for the bitcoin-cli.exe binary. This is
due to it not having a .reloc section, which is stripped by default by
the mingw-w64 ld we use for gitian builds. A good summary of issues with
ld and mingw-w64 is available in this thread:
https://sourceware.org/bugzilla/show_bug.cgi?id=19011.

All other Windows binaries that we distribute (bitcoind, bitcoin-qt,
bitcoin-wallet, bitcoin-tx and test_bitcoin) do not suffer this issue,
and currently having working ASLR. This is due to them exporting
(inadvertent or not) libsecp256k1 symbols, and, as a result, the .reloc
section is not stripped by ld.

This change is a temporary workaround, also the same one described here:
https://www.kb.cert.org/vuls/id/307144/, that causes main() to be
exported. Exporting a symbol will mean that the .reloc section is not
stripped, and ASLR will function correctly.

src/bitcoin-cli.cpp

@@ -551,11 +551,19 @@ static int CommandLineRPC(int argc, char *argv[])
     return nRet;
 }
 
-int main(int argc, char* argv[])
-{
 #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[])
+{
     util::WinCmdLineArgs winArgs;
     std::tie(argc, argv) = winArgs.get();
+#else
+int main(int argc, char* argv[])
+{
 #endif
     SetupEnvironment();
     if (!SetupNetworking()) {