MFH: r396167

Disable use of SSE instructions in Xorg's xf86SlowBcopy() function.

When such instructions are used to copy data from/to mapped video
memory, some hypervisors (e.g. KVM, Microsoft Hyper-V) can generate
SIGILL or SIGBUS exceptions, causing Xorg to crash.

Reported by: nogcjx@fastmail.fm
Approved by: ports-secteam (feld)
PR: 202643