Support 32bit pointers in 64bit architectures

[nshp]

Several architectures/compilers have a weird compatibility mode where they'll emit 64bit instructions (and 64bit ELF types, relocations, and so on), but use 32bit sign-extended pointers. The most notable example is Intel x32 (e.g. gcc -mx32), but MIPS64 and AArch64 also have similar features, and I've seen all of them. Basically, the ISA is 64bit, but longs and pointers stored in memory are 32 bits. Whenever a pointer is loaded, it's sign-extended to 64 bits. MIPS64 in system mode also uses a high memory range (> 0xffffffff800000 I believe) so that kernel pointers get sign-extended meaningfully (e.g. sx.q(0x80c00000) => 0xffffffff80c00000). Stack analysis completely gives up when it sees a sign extension on the stack pointer, that's the biggest problem for me. The next biggest issue is that I can't use actual pointer types anywhere (because they're the wrong size), so type propagation and xreffing (e.g. a global pointer to a function) don't work out well.

I'm not really sure what the best solution would look like, but just wanted to note that I've seen this in 5 very different places now and haven't had a great workaround for binja.