build: fix ASLR for bitcoin-cli on Windows
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.

Github-Pull: #18702
Rebased-From: 315a4d36f716341a38bc4e4de8630b3246d27dbc

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