Merge bitcoin#18702: build: fix ASLR for bitcoin-cli on Windows

315a4d3 build: fix ASLR for bitcoin-cli on Windows (fanquake)

Pull request description:

  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.

  Ultimately, this will be fixed by using a newer version of binutils (that has this [change](https://sourceware.org/git/gitweb.cgi?p=binutils-gdb.git;a=commit;h=dc9bd8c92af67947db44b3cb428c050259b15cd0)). Whether that's through bumping our gitian distro, or Guix.

  Related to bitcoin#18629, which has a bunch of additional information in the PR description. If you would like to verify whether or not ASLR is indeed working, with or without this change. One easy way to check is using a tool like [VMMap](https://docs.microsoft.com/en-us/sysinternals/downloads/vmmap).

  Here are the memory mappings for the 0.20.0rc1 `bitcoind.exe` and `bitcoin-cli.exe` binaries. You'll notice that over machine restarts, even though the image is marked `(ASLR)` (which I assume may be due to the header bit being set), no ASLR is actually occuring for `bitcoin-cli.exe`:

  #### bitcoind.exe

  ![bitcoind-1](https://user-images.githubusercontent.com/863730/79678203-74065c80-822b-11ea-90bc-9c883d0aeefa.png)

  ![bitcoind-2](https://user-images.githubusercontent.com/863730/79678204-7668b680-822b-11ea-9263-3e7ba22f904c.png)

  ![bitcoind-3](https://user-images.githubusercontent.com/863730/79678206-7963a700-822b-11ea-972f-af31a514b9b4.png)

  #### bitcoin-cli.exe

  ![bitcoin-cli-1](https://user-images.githubusercontent.com/863730/79678208-7ec0f180-822b-11ea-8480-a4b5d1762945.png)

  ![bitcoin-cli-2](https://user-images.githubusercontent.com/863730/79678213-81bbe200-822b-11ea-964d-994f58ff12b0.png)

  ![bitcoin-cli-3](https://user-images.githubusercontent.com/863730/79678215-84b6d280-822b-11ea-9cd6-fee2e239c003.png)

ACKs for top commit:
  dongcarl:
    ACK 315a4d3
  laanwj:
    ACK 315a4d3

Tree-SHA512: 95f4dc15420ed9bcdeacb763e11c3c7e563eec594a172746fa0346c13f97db3a8769357dffc89fea1e57ae67133f337b1013a73b584662f5b6c4d251ca20a2b1

Author: laanwj

src/bitcoin-cli.cpp

@@ -558,7 +558,16 @@ static int CommandLineRPC(int argc, char *argv[])
     return nRet;
 }
 
+#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
 {
     RegisterPrettyTerminateHander();
     RegisterPrettySignalHandlers();