Merge bitcoin#20939: build: fix RELOC_SECTION security check for bitc…

…oin-util

c061800 build: fix RELOC_SECTION security check for bitcoin-util (fanquake)

Pull request description:

  The binutils we use for gitian builds strips the reloc section from
  Windows binaries, which breaks ASLR. As a temporary workaround, export
  main(). This is the same workaround as bitcoin#18702 (bitcoin-cli), and will
  fix the currently failing security check:
  ```bash
  + make -j1 -C src check-security
  make: Entering directory '/home/ubuntu/build/bitcoin/distsrc-x86_64-w64-mingw32/src'
  Checking binary security...
  bitcoin-util.exe: failed RELOC_SECTION
  make: *** [check-security] Error 1
  ```

  Relevant upstream issue:
  https://sourceware.org/bugzilla/show_bug.cgi?id=19011

ACKs for top commit:
  dongcarl:
    ACK c061800
  laanwj:
    ACK c061800

Tree-SHA512: a1a4da0b2cddfc377190b9044a04f42a859ca79f11ce2c2ab4c3d066a2786c34d5446d75f8eec634f308d2d3349ebbd7c9f76dcaebeeb28e471c829851592367

src/bitcoin-util.cpp

@@ -181,7 +181,16 @@ static int CommandLineUtil(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
 {
     SetupEnvironment();