Failing testcase on FSTP with 80bit float

[kyle-elliott-tob]

https://github.com/lifting-bits/remill/actions/runs/19861155546/job/56911369610

Introduced in 9816ccc, the bug likely was caused by the recent XED update, as it previously passed on CI. Needs bissecting to determine the exact change that caused it

Possible cause commit 380578b