[LFI][libunwind] Avoid writing to reserved registers on the `aarch64_lfi` target (#192739)

The registers `x25`, `x26`, `x27`, and `x28` are reserved for the
`aarch64_lfi` target and should not be written/used. See
`llvm/docs/LFI.rst` for more details. They contain fixed constants or
temporaries used as part of assembly-level rewrites of load/store
instructions. The rewriter will reject programs that attempt to write to
these registers. It is technically OK to save/read them (in
`UnwindRegistersSave.S`), but there is no point since there is no need
to restore them (and to maintain LFI's security invariant, they in fact
cannot be written).

GitOrigin-RevId: 2b63e5e75dbadc30218e41c8113a40acbc625334