gdb: make regcache::debug_print_register return a string

Rename the method to `register_debug_string`.

This makes it easier to introduce `target_debug_printf` in a subsequent
patch.

Change-Id: I5bb2d49476d17940d503e66f40762e3f1e3baabc
Approved-By: Tom Tromey <tom@tromey.com>
3 files changed