[gdb/tdep] Fix rip-relative insn handling in amd64_get_used_input_int_reg

I wanted to add a unit test for an an rip-relative amd64 insn, so I did:
...
$ gcc -fPIE hello.c
...
and used an rip-relative insn from main:
...
  4005db:       48 8d 3d 1e 00 00 00    lea    0x1e(%rip),%rdi
...

While writing the unit test, I found that amd64_get_used_input_int_reg returns
rbp as input register.

Fix this by using rip_relative_p in amd64_get_used_input_int_reg to handle
this case.

Tested on x86_64-linux.