llvm12: Disable LLDB on non-x86 architectures

The 12.0.0 branch point hit point in the rework of FreeBSD debug targets
were non-x86 architectures are broken. For now, simply disable LLDB
builds on those platforms.

PR: 254082
(cherry picked from commit cc0bbd86088c545607a8585256adb8e3cfb132ad)