Auto merge of #37672 - japaric:msp430, r=alexcrichton

enable the MSP430 LLVM backend

to let people experiment with this target out of tree.

The MSP430 architecture is used in 16-bit microcontrollers commonly used
in Digital Signal Processing applications.

---

How this was tested:

Declaring a custom target with the following specification:

``` json
{
  "arch": "msp430",
  "data-layout": "e-m:e-p:16:16-i32:16:32-a:16-n8:16",
  "executables": true,
  "linker": "msp430-gcc",
  "llvm-target": "msp430",
  "max-atomic-width": 0,
  "no-integrated-as": true,
  "os": "none",
  "panic-strategy": "abort",
  "relocation-model": "static",
  "target-endian": "little",
  "target-pointer-width": "16"
}
```

And this minimal file:

``` rust

pub fn start() -> ! {
    loop {}
}

trait Copy {}

trait Sized {}
```

Produces the following object files:

```
$ rustc --target=msp430 --emit=obj foo.rs

$ msp430-objdump -Cd foo.o

foo.o:     file format elf32-msp430

Disassembly of section .text.start:

00000000 <start>:
   0:   21 83           decd    r1
   2:   00 3c           jmp     $+2             ;abs 0x4
   4:   00 3c           jmp     $+2             ;abs 0x6
   6:   ff 3f           jmp     $+0             ;abs 0x6

$ rustc --target=msp430 --emit=obj foo.rs -O

$ msp430-objdump -Cd foo.o

foo.o:     file format elf32-msp430

Disassembly of section .text.start:

00000000 <start>:
   0:   ff 3f           jmp     $+0             ;abs 0x0
```

---

r? @alexcrichton
~~TODO get this working with Makefiles so nightly releases include this backend~~
~~TODO measure the increase in binary size~~ +187KiB (+0.47%)
~~FIXME --emit=obj produces empty object files~~

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