Argon2: fix build with -march=x86-64-v3/v4 and XOP targets

[flatstik]

Fixes #1867 (the build error also reported in #1871). Alternative to #1890, opened at @puleglot's request.

When squashing: the first commit message says Fixes #1871, please change that to Refs #1871, since #1871 is mainly about the Ubuntu 22.04 CI runner, which this doesn't touch.

blamka-round-opt.h picked its SSE2, AVX2 or AVX-512 definitions only from the compiler flags. With global flags such as -march=x86-64-v3 or -march=x86-64-v4 (Ubuntu 26.04, distribution builds, -march=native), opt_sse2.c, and with v4 also opt_avx2.c, got definitions that don't match their code, and the build failed.

Changes:

• opt_sse2.c defines ARGON2_BLAMKA_SSE2 and opt_avx2.c defines ARGON2_BLAMKA_AVX2 before including the header, and the header uses that to select the variant. Any other includer still gets the old selection by compiler flags.
• No new compiler options, so this works with old compilers (no -mno-avx512f needed) and also covers the plain objects used by NOASM=1 builds.
• With XOP targets (-march=bdver*), the header skipped its _mm_roti_epi64 fallback, but the include that declares the intrinsic was commented out. The #include <x86intrin.h> that is already guarded by __XOP__ && (__GNUC__ || __clang__) is enabled again; MSVC and the Windows driver never take that branch.

Testing:

• opt_sse2.c, opt_avx2.c -mavx2 and opt_avx2.c without -mavx2 (as in NOASM=1) compile without errors or warnings (-Wall -Wextra) with GCC 10, 12, 14.2 and Clang 19.1.7, for default, x86-64-v2, x86-64-v3, x86-64-v4, bdver2, bdver4, znver4 (not known to GCC 10), skylake-avx512 and native. On master, the v3, v4, bdver2, bdver4, znver4, skylake-avx512 and native builds (on an AVX2 host) fail.
• Objects built with the default flags (and with NOASM=1-style plain objects) are byte-identical to master.
• A small harness hashes with argon2id_hash_raw through the AVX2 path, the SSE2 path, and the portable code (forced with ld --wrap on x86-64, where HasSSE2() is always 1). All three give identical hashes, which also match an unmodified master build, with GCC 14 and Clang 19 for default and -march=x86-64-v3 flags. The Argon2i/Argon2id self-tests pass through the AVX2 and SSE2 paths with GCC 10, GCC 14 and Clang 19 for default and -march=x86-64-v3 flags.
• The v4 and XOP builds are compile-tested only; I have no such CPU.

[idrassi]

Thanks, I prefer this approach.