[gdb/tdep] Refactor amd64_get_unused_input_int_reg, part 1

[gdb/tdep] Refactor amd64_get_unused_input_int_reg, part 1

While reading amd64_get_unused_input_int_reg, I noticed that it first asserts,
then throws an internal_error if no unused register can be found.

Looking at the documentation of gdbarch_displaced_step_copy_insn, it seems
that a failure can be indicated less abruptly, by returning a nullptr.

Fix this by:
- returning -1 in case of failure to find an unused register in
  amd64_get_unused_input_int_reg, and
- propagating this to amd64_displaced_step_copy_insn.

Tested on x86_64-linux.