And add a check in the python script that the binary given to `--qemu`
actually exists. Otherwise you get a generic Python error:
```
# .---command stderr------------
# | Traceback (most recent call last):
# | File "/home/david.spickett/modules-llvm-project/libcxx/utils/qemu_baremetal.py", line 70, in <module>
# | exit(main())
# | File "/home/david.spickett/modules-llvm-project/libcxx/utils/qemu_baremetal.py", line 66, in main
# | os.execvp(qemu_commandline[0], qemu_commandline)
# | File "/usr/lib/python3.8/os.py", line 568, in execvp
# | _execvpe(file, args)
# | File "/usr/lib/python3.8/os.py", line 610, in _execvpe
# | raise last_exc
# | File "/usr/lib/python3.8/os.py", line 601, in _execvpe
# | exec_func(fullname, *argrest)
# | FileNotFoundError: [Errno 2] No such file or directory
# `-----------------------------
# error: command failed with exit status: 1
```
When it tries to run the entire command later.
For the builder, it's only ever going to use qemu-system-arm so error at
config time if it's not there.
VSCode's Pylance extension informed me, and text searching confirmed,
that these imports are unused. I believe we should be able to remove
them harmlessly.
This is useful when trying to test libc++/libc++abi/libunwind against a
baremetal enviroment (e.g. picolibc).
See also https://reviews.llvm.org/D155521
