Add basic support for x86_32 architecture

[alexcrichton]

This corresponds to the Rust x86_64-unknown-linux-gnux32 target,
notably that the architecture listed is x86_64 but the word size is 32

[bjorn3]

x32 may be dropped from the Linux kernel in the future: https://lkml.org/lkml/2018/12/10/1145

[philipc]

I prefer if the minimum support added is enough to pass the test at

object/tests/round_trip/mod.rs

Line 229 in 55a50c8

fn elf_any() {

[alexcrichton]

Certainly! Some small testing locally appeared that relocations for x86_32 are encoded in the same way as x86_64, although I am not certain about this. Do you know of a location to confirm that?

[philipc]

LLVM agrees with that: https://github.com/llvm/llvm-project/blob/e712a92b8fe706c4a736748af324382850d1515c/llvm/lib/Target/X86/MCTargetDesc/X86ELFObjectWriter.cpp#L334-L339

Also, I'm not sure that x86_32 is the correct name for this; to me, that is the same as i386. I've seen this referred to as x32, but I don't know what the target triple uses. Edit: you gave the target triple..

[alexcrichton]

Yeah I seem to see a lot of various names, but presumably keeping "x32" somewhere in the name is most reasonable? Maybe X32 alone? Or X86_64_X32 (although that's a mouthful...)? Or just X86_X32?

[philipc]

Let's go with X86_64_X32. The architecture really is X86_64, but the problem is we're assuming the address size from the architecture, which is convenient for most cases but doesn't work here.

[alexcrichton]

Sounds good! Pushed up

[philipc]

Thanks!