[LFI][libunwind] Omit unsuppported stg/za instructions for LFI target (#209392)

Under the LFI target the SVE `za` are not currently supported, and
neither is the `stg` instruction. When included in an LFI program they
cause verification failures due to being unsupported instructions. This
patch omits them from compilation when targeting LFI.

GitOrigin-RevId: c4026e4350fd4d70692f205aff8764055a8b4ce1