Document QEMU persistent hook on mipsel

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`.

utils/qemu_persistent_hook/mipsel_read_into_a0.c

@@ -0,0 +1,35 @@
+#define TARGET_MIPS
+#include "../../qemu_mode/qemuafl/qemuafl/api.h"
+
+#include <stdio.h>
+#include <string.h>
+
+#define g2h(x) ((void *)((unsigned long)(x) + guest_base))
+#define h2g(x) ((uint64_t)(x) - guest_base)
+
+void afl_persistent_hook(struct mips_regs *regs, uint64_t guest_base,
+                         uint8_t *input_buf, uint32_t input_buf_len) {
+
+  // In this example the register $a0 is pointing to the memory location
+  // of the target buffer, and the length of the input is in $a1.
+  // This can be seen with a debugger, e.g. gdb (and "disass main")
+
+  printf("Placing input into 0x%lx\n", regs->a0);
+
+  if (input_buf_len > 1024) input_buf_len = 1024;
+  memcpy(g2h(regs->a0), input_buf, input_buf_len);
+  regs->a1 = input_buf_len;
+
+}
+
+#undef g2h
+#undef h2g
+
+int afl_persistent_hook_init(void) {
+
+  // 1 for shared memory input (faster), 0 for normal input (you have to use
+  // read(), input_buf will be NULL)
+  return 1;
+
+}
+