prepare for rustc for mips-gnu, mipsel-gnu, ppc64-gnu and ppc64el-gnu

[japaric]

Just (*) by setting these env vars I could can use this command:

$ configure --enable-llvm-static-stdcpp --enable-rustbuild --host=mips-unknown-linux-gnu
$ make

to build rustc for these targets.

(*) for ppc64, I additionally installed g++ for ppc64 because I didn't want to mess with CFLAGS
(i.e. -m64).

I tested building rustc for each target. Then tested, under QEMU, that each (**) cross compiled
rustc could, itself, compile the "smallest hello" program (see code below). And, finally, I executed the smallest hello binary that was "natively" (under QEMU, withouth --target flag) compiled by the foregin rustc.

The commands looked like this:

$ uname -a
Linux 7d6aa2c97ca4 4.6.4 #1 SMP Fri Jul 22 11:55:10 PET 2016 x86_64 x86_64 x86_64 GNU/Linux

$ export QEMU_LD_PREFIX=/usr/powerpc64-linux-gnu

# Note: transparent QEMU emulation via the qemu-user-static and binfmt-support packages
$ rustc -Vv
rustc 1.13.0-dev (eaf71f8d1 2016-08-25)
binary: rustc
commit-hash: eaf71f8d1034f16140791f566cab3f3c9a0bf96a
commit-date: 2016-08-25
host: powerpc64-unknown-linux-gnu
release: 1.13.0-dev

$ rustc -C linker=powerpc64-linux-gnu-gcc-5 smallest-hello.rs

$ file smallest-hello
puts: ELF 64-bit MSB shared object, 64-bit PowerPC or cisco 7500, version 1 (SYSV), dynamically linked, interpreter /lib64/ld64.so.1, for GNU/Linux 3.2.0, BuildID[sha1]=600eca53c0d784de9e680bc9e52a0e84af25acb9, not stripped

$ ./smallest-hello
Hello, world!

// smallest-hello.rs
#![feature(start)]
#![feature(lang_items)]
#![feature(no_core)]
#![no_core]

#[link(name = "c")]
extern {
    fn puts(_: *const u8);
}

#[start]
fn start(_: isize, _: *const *const u8) -> isize {
    unsafe {
        let msg = b"Hello, world!\0";
        puts(msg as *const _ as *const u8);
    }
    0
}


#[lang = "copy"]
trait Copy {}

#[lang = "sized"]
trait Sized {}

(**) I couldn't test the ppc64el target because qemu-ppc64el segfaulted with every binary I threw at
it (I even tried this C program: int main() { return 0; }). But rustc did successfully cross compile for that target.

r? @alexcrichton

[alexcrichton]

Nice!

[alexcrichton]

Ok, I've built a docker container and pushed it, but I now realize it may not be too useful until we get libc-test working...

[japaric]

FWIW, libc-ctest passes on mips and on mipsel. libc-ctest doesn't compile for ppc64 and qemu for ppc64el is kaput so I didn't even try to compile libc-ctest for it.

[alexcrichton]

Ok the next steps here I think are:

1. Land support in rust-lang/rust for these targets
2. Get the bootstrap of new compilers working for these targets in the most recent docker containers
3. Conclude we have the budget for this
4. Figure out how to schedule all the new nightlies
5. Add entries and start building nightlies