[gdb/tdep] Refactor amd64_get_unused_input_int_reg, part 3

[gdb/tdep] Refactor amd64_get_unused_input_int_reg, part 3

While reading amd64_get_unused_input_int_reg, I noticed that it avoids picking
RSP, which has to do with how the result of the only call to it is going to be
used.

Likewise for picking a register in the RAX ... RDI range.

Fix this by:
- adding an allowed_regs_mask parameter to amd64_get_unused_input_int_reg, and
- properly documenting the value of the corresponding argument in fixup_riprel.

No functional changes.

Tested on x86_64-linux.