x86/CPU: Fix FPDSS on Zen1

commit e55d98e7756135f32150b9b8f75d580d0d4b2dd3 upstream.

Zen1's hardware divider can leave, under certain circumstances, partial
results from previous operations.  Those results can be leaked by
another, attacker thread.

Fix that with a chicken bit.

BUG=b/506403795
TEST=presubmit
RELEASE_NOTE=Fixed CVE-2026-31628 in the Linux kernel.

Change-Id: I267aa498c3210878d35db36e28938415eda3eda7
Signed-off-by: Borislav Petkov (AMD) <bp@alien8.de>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: Angel Adetula <angeladetula@google.com>
Reviewed-on: https://cos-review.googlesource.com/c/third_party/kernel/+/148012
Tested-by: Cusky Presubmit Bot <presubmit@cos-infra-prod.iam.gserviceaccount.com>
Reviewed-by: Robert Kolchmeyer <rkolchmeyer@google.com>