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`.
