[GDB] Use "gdb_arch_list_lookup_by_info" in "arc_gdbarch_init"

Using `gdb_arch_list_lookup_by_info()` during the `arc_gdbarch_init()` will allow us to reuse the _gdbarch_ object whenever an inferior is restarted.

Apparently, `gdb_arch_list_lookup_by_info()` allows reusing the _gdbarch_ objects when _arch_, _abi_, _target description_, and optional _other_ stuffs remain the same.

One can look into other targets to get some idea.