support ARCH and CROSS_COMPILE variables as arguments

Now, it supports only ${ARCH} and ${CROSS_COMPILE} as environment variables, which makes users confused because usually they consider environment variables arguments as belows:

```
$ ARCH=arm ./qemu-linux # environment variables: it works now.
$ ./qemu-linux ARCH=arm # arguemnts: it does not work.
```

Start at the qemu-linux command entry point and inspect how ARCH and CROSS_COMPILE are currently read from the environment. Extend argument handling so the shown ARCH=arm form is accepted while preserving existing environment-variable behavior. Done means both examples work; add or update tests if the repository provides them.

Written by the indexing model from the issue text.