PNaCl uses the ILP32 data model, even on x86-64 (cf. the x32 ABI).
Reactor instead always uses LP64 on 64-bit.

The main technical difference is the ILP32 on x86-64 used the address-
size override prefix (0x67) for every instruction that accesses
memory, which makes it look at only the lower 32-bit part of base and
index registers. Note this only works in a sandbox-like environment,
where memory is allocated in the lower 4 GiB range, such as PNaCl.

Bug: b/179832693
Change-Id: I068ddd12b1827e3babc49a06ddf26b932d2082c5
Reviewed-on: https://swiftshader-review.googlesource.com/c/SwiftShader/+/55468
Kokoro-Result: kokoro <noreply+kokoro@google.com>
Tested-by: Nicolas Capens <nicolascapens@google.com>
Reviewed-by: Alexis Hétu <sugoi@google.com>