lib/crypto: poly1305: Restore dependency of arch code on !KMSAN

Restore the dependency of the architecture-optimized Poly1305 code on
!KMSAN.  It was dropped by commit b646b782e522 ("lib/crypto: poly1305:
Consolidate into single module").

Unlike the other hash algorithms in lib/crypto/ (e.g., SHA-512), the way
the architecture-optimized Poly1305 code is integrated results in
assembly code initializing memory, for several different architectures.
Thus, it generates false positive KMSAN warnings.  These could be
suppressed with kmsan_unpoison_memory(), but it would be needed in quite
a few places.  For now let's just restore the dependency on !KMSAN.

Note: this should have been caught by running poly1305_kunit with
CONFIG_KMSAN=y, which I did.  However, due to an unrelated KMSAN bug
(https://lore.kernel.org/r/20251022030213.GA35717@sol/), KMSAN currently
isn't working reliably.  Thus, the warning wasn't noticed until later.

Fixes: b646b782e522 ("lib/crypto: poly1305: Consolidate into single module")
Reported-by: syzbot+01fcd39a0d90cdb0e3df@syzkaller.appspotmail.com
Closes: https://lore.kernel.org/r/68f6a48f.050a0220.91a22.0452.GAE@google.com/
Reported-by: Pei Xiao <xiaopei01@kylinos.cn>
Closes: https://lore.kernel.org/r/751b3d80293a6f599bb07770afcef24f623c7da0.1761026343.git.xiaopei01@kylinos.cn/
Reviewed-by: Ard Biesheuvel <ardb@kernel.org>
Link: https://lore.kernel.org/r/20251022033405.64761-1-ebiggers@kernel.org
Signed-off-by: Eric Biggers <ebiggers@kernel.org>

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