Auto merge of #38726 - japaric:sparc64, r=sanxiyn

Auto merge of #38726 - japaric:sparc64, r=sanxiyn

sparc64-linux support

This is built on top of #38656 and depends on rust-lang/libc#483

Hello world works.

The libc-test test suite passes.

`panic!` doesn't fully work:

```
$ qemu-sparc64-static ./panic
thread 'main' panicked at 'explicit panic', panic.rs:1
note: Run with `RUST_BACKTRACE=1` for a backtrace.
Illegal instruction (core dumped)
```

Backtraces don't work either, probably related to the previous point:

```
$ export RUST_BACKTRACE=1
$ qemu-sparc64-static ./panic
thread 'main' panicked at 'explicit panic', panic.rs:1
stack backtrace:
Illegal instruction (core dumped)
```

r? @alexcrichton

@jakllsch Does panicking / backtraces work on sparc64-netbsd?

cc @glaubitz