Merge #128

128: Add qemu example and run in CI r=japaric a=sekineh

As @japaric has suggested I ported `lm3s6965evb/src/main.rs` into `example/qemu.rs`.

All errors were fixed, ~but qemu never exits automatically~ `qemu` prints text and exits.
```
sekineh@sekineh-VirtualBox:~/cortex-m-rt_me$ cargo run --example qemu --target thumbv7m-none-eabi
   Compiling cortex-m-rt v0.6.3 (file:///home/sekineh/cortex-m-rt_me)                                            
    Finished dev [unoptimized + debuginfo] target(s) in 0.17s
     Running `qemu-system-arm -cpu cortex-m3 -machine lm3s6965evb -nographic -semihosting-config enable=on,target=native -kernel target/thumbv7m-none-eabi/debug/examples/qemu`
x = 42
sekineh@sekineh-VirtualBox:~/cortex-m-rt_me$
```
## supported targets

- thumbv6m-none-eabi
- thumbv7m-none-eabi

## code size

~code size is 0, which is apparently wrong:~
```
sekineh@sekineh-VirtualBox:~/cortex-m-rt_me$ size target/thumbv7m-none-eabi/debug/examples/qemu
   text	   data	    bss	    dec	    hex	filename
  10776	      0	      0	  10776	   2a18	target/thumbv7m-none-eabi/debug/examples/qemu
```

## code size (original)
(deleted; different compiler version)

Co-authored-by: Hideki Sekine <sekineh@me.com>

0 files changed, 0 insertions, 0 deletions