[LFI][X86] Add memory access rewrites (#227511)
This adds memory access rewriting for the x86-64 LFI target using `%gs`
as the base register for rewrites. By using 32-bit register combined
with the sandbox base in `%gs`, memory accesses can be rewritten to be
confined to an LFI sandbox with little overhead. See the documentation
in `LFI.md` for details on exact rewrite sequences.
