x.crypto.slhdsa: add support on OpenBSD using OpenSSL 3.5

[lcheylus]

OpenSSL version 3.5 available on OpenBSD 7.8 (current release) via ports => add flags in vlib/x/crypto/slhdsa/slhdsa.c.v to use it for openbsd.

Tests OK with tcc / clang 19

$ uname -mrs
OpenBSD 7.8 amd64
$ eopenssl35 version
OpenSSL 3.5.4 30 Sep 2025 (Library: OpenSSL 3.5.4 30 Sep 2025)

$ ./v -d has_modern_openssl -cc tcc test vlib/x/crypto/slhdsa/
---- Testing... ----------------------------------------------------------------------------------------------------------------------------
 OK    [1/4] C:   797.8 ms, R:    41.026 ms vlib/x/crypto/slhdsa/slhdsa_sigver_test.v
 OK    [2/4] C:   792.5 ms, R:  1525.497 ms vlib/x/crypto/slhdsa/usage_test.v
 OK    [3/4] C:   922.6 ms, R:  5246.069 ms vlib/x/crypto/slhdsa/base_test.v
 OK    [4/4] C:   842.5 ms, R:  9869.963 ms vlib/x/crypto/slhdsa/slhdsa_siggen_test.v
--------------------------------------------------------------------------------------------------------------------------------------------
Summary for all V _test.v files: 4 passed, 4 total. Elapsed time: 10756 ms, on 3 parallel jobs. Comptime: 3355 ms. Runtime: 16682 ms.

$ ./v -d has_modern_openssl -cc clang test vlib/x/crypto/slhdsa/
---- Testing... ----------------------------------------------------------------------------------------------------------------------------
 OK    [1/4] C:  1583.2 ms, R:    41.451 ms vlib/x/crypto/slhdsa/slhdsa_sigver_test.v
 OK    [2/4] C:  1475.7 ms, R:  1404.662 ms vlib/x/crypto/slhdsa/usage_test.v
 OK    [3/4] C:  1654.8 ms, R:  5548.034 ms vlib/x/crypto/slhdsa/base_test.v
 OK    [4/4] C:  1586.4 ms, R: 11003.026 ms vlib/x/crypto/slhdsa/slhdsa_siggen_test.v
--------------------------------------------------------------------------------------------------------------------------------------------
Summary for all V _test.v files: 4 passed, 4 total. Elapsed time: 12635 ms, on 3 parallel jobs. Comptime: 6300 ms. Runtime: 17997 ms.

[lcheylus]

cc @blackshirt (author of the initial V implementation for x.crypto.slhdsa).

[blackshirt]

cc @blackshirt (author of the initial V implementation for x.crypto.slhdsa).

I have not yet play with openbsd machine now,
But, adds support for another os ia a good thing The PR looks fine to me.

Thanks @lcheylus