qemuafl now supports persistent mode for *mipsel* targets. This patch
changes the `SYS` variable tests and runs the persistent qemu_mode
tests for *mipsel* as well.

This also adds an optional environment variable called `CPU_TARGET_CC`
that you can pass to the `./test-qemu-mode.sh` test script.

This allows you to specify a cross-compiler for the target platform. The
test script then it uses to compile `test-instr.c` and `test-compcov.c`.

Example usage:

```
CPU_TARGET_CC=mipsel-linux-gnu-cc CPU_TARGET=mipsel ./test-qemu-mode.sh
```

The output should look something like this:

```
[*] Using environment variable CPU_TARGET=mipsel for SYS
[*] starting AFL++ test framework ...
[*] Testing: qemu_mode
[*] Using mipsel-linux-gnu-cc as compiler for target
[*] running afl-fuzz for qemu_mode, this will take approx 10 seconds
[+] afl-fuzz is working correctly with qemu_mode
[*] running afl-fuzz for qemu_mode AFL_ENTRYPOINT, this will take approx 6 seconds
[+] afl-fuzz is working correctly with qemu_mode AFL_ENTRYPOINT
[-] not an intel or arm platform, cannot test qemu_mode compcov
[-] not an intel or arm platform, cannot test qemu_mode cmplog
[*] running afl-fuzz for persistent qemu_mode, this will take approx 10 seconds
[+] afl-fuzz is working correctly with persistent qemu_mode
[+] persistent qemu_mode was noticeable faster than standard qemu_mode
[*] running afl-fuzz for persistent qemu_mode with AFL_QEMU_PERSISTENT_EXITS, this will take approx 10 seconds
[+] afl-fuzz is working correctly with persistent qemu_mode and AFL_QEMU_PERSISTENT_EXITS
[+] persistent qemu_mode with AFL_QEMU_PERSISTENT_EXITS was noticeable faster than standard qemu_mode
[-] we cannot test qemu_mode unsigaction library (32 bit) because it is not present
[+] qemu_mode unsigaction library (64 bit) ignores signals
[*] 1 test cases completed.
[-] not all test cases were executed
[+] all tests were successful :-)
```

This adds a new persistent hook library `mipsel_read_into_a0.c`. With
it, you can test the persistent hook on the *mipsel* architecture.

I'm also updating the README in `utils/qemu_persistent_hook` and
Makefile and explain how to test the persistent hook on *mipsel*.

This all works thanks to qemuafl already having the correct CPU struct
for *mipsel* in `qemuafl/api.h`.

This patch also updates the root `.gitignore` file to ignore the two
test binaries `test` and `mipsel_test`.