Add crate build test for `thumb*` targets. [IRR-2018-embedded]
## Summary
This PR adds `run-make` test that compiles `cortex-m` crate for all supported `thumb*-none-*` targets using `cargo` and stage2 `rustc`.
- Supported `thumb*-none-*` targets:
- thumbv6m-none-eabi (Bare Cortex-M0, M0+, M1)
- thumbv7em-none-eabi (Bare Cortex-M4, M7)
- thumbv7em-none-eabihf (Bare Cortex-M4F, M7F, FPU, hardfloat)
- thumbv7m-none-eabi (Bare Cortex-M3)
## How to run & Example output
I tested locally and all targets succeeded like below:
```
./x.py clean
./x.py test --target thumbv6m-none-eabi,thumbv7em-none-eabi,thumbv7em-none-eabihf,thumbv7m-none-eabi src/test/run-make
```
```
Check compiletest suite=run-make mode=run-make (x86_64-unknown-linux-gnu -> thumbv6m-none-eabi)
running 5 tests
.....
test result: ok. 5 passed; 0 failed; 0 ignored; 0 measured; 0 filtered out
```
## How to re-run
Remove `stamp` file for the test run.
```
rm build/x86_64-unknown-linux-gnu/test/run-make/thumb-none-cortex-m/stamp
```
Then run `test`
```
./x.py test --target thumbv6m-none-eabi,thumbv7em-none-eabi,thumbv7em-none-eabihf,thumbv7m-none-eabi src/test/run-make
(snip)
running 5 tests
iiii.
test result: ok. 1 passed; 0 failed; 4 ignored; 0 measured; 0 filtered out
```
## Artifacts
You can examine the artifacts under the directory below:
```
sekineh@sekineh-VirtualBox:~/rustme10$ ls -l build/x86_64-unknown-linux-gnu/test/run-make/thumb-none-cortex-m/thumb-none-cortex-m/
total 4
drwxrwxr-x 7 sekineh sekineh 4096 8月 14 22:40 cortex-m
```
where `build/x86_64-unknown-linux-gnu/test/run-make/thumb-none-cortex-m/thumb-none-cortex-m/` is came from TMPDIR variable.
## Other notes
For `test.rs` modification, I used the same logic as:
- https://github.com/rust-lang/rust/blame/d8b3c830fbcdd14d085209a8dcc3399151f3286a/src/bootstrap/dist.rs#L652-L657
```
if builder.no_std(target) == Some(true) {
// the `test` doesn't compile for no-std targets
builder.ensure(compile::Std { compiler, target });
} else {
builder.ensure(compile::Test { compiler, target });
}
```
It is a useful snippet when adding `no_std` support to `src/bootstrap` code.
CC @kennytm @jamesmunns @nerdyvaishali
8b923a19ed